[rubygems/rubygems] Revert adding loaded specs to `Gem::Specification.stubs` and `Gem::Specification.stubs_for`

The rationale is that:

* The change has caused realworld issues. See for example
https://github.com/ruby/did_you_mean/issues/117 and specifically [this
comment](https://github.com/ruby/did_you_mean/issues/117#issuecomment-482733159)
for a great explanation of the issue it caused for `did_you_mean`.

* The change also causes problems for our development workflows. For
example, because of it, our `bundler` specs cannot currently be run with
`bin/rake` and we have to use `bin/rspec` or `bin/parallel_spec`
directly. The explanation for this is:

  - Our specs install test dependencies to `tmp` before running specs.
  - `rake` is one of these test dependencies.
  - Before installing each test dependency, we check whether it has
  matching installed specs: 2bbcdcde08/bundler/spec/support/rubygems_ext.rb (L109-L114).

  - Normally, if `rake` has not yet been installed to `tmp`, this check
  fails and `rake` is installed, but since the loaded specs are now
  added to `Gem::Specification.stubs` and `rake`'s specification _is_
  loaded because we're running through `bin/rake`, the check incorrectly
  assumes that `rake` is already installed to `tmp` and skips
  installation.
  - At a later point the specs check whether `rake` is actually
  installed and fail if it's not: 2bbcdcde08/bundler/spec/support/builders.rb (L372-L383)

Essentially, both of the issues are the same. If at runtime we change
the location of gems, we'll _want_ to not consider loaded specifications
when dealing with the new gem location, because the loaded
specifications have not been loaded from there. Loaded specifications is
something different from installed stub specifications and those should
not be mixed.

The PR still seemed to have fixed an issue, so I did my archaeology job
and investigated the original issue to double check if reverting is ok.
The logs for the original error can be found here:
https://ci.appveyor.com/project/rubygems/rubygems/build/1172/job/ogubyucpljcv22ux.

So I installed ruby 2.4.4, checked out the commit reference before the
offending PR, and the exact error reproduced. 🎉

```
$ rake test
/home/deivid/Code/rubygems/lib/rubygems/resolver.rb:231:in `search_for': Unable to resolve dependency: user requested 'bundler (= 1.16.2)' (Gem::UnsatisfiableDependencyError)
  from /home/deivid/Code/rubygems/lib/rubygems/resolver.rb:283:in `block in sort_dependencies'
  from /home/deivid/Code/rubygems/lib/rubygems/resolver.rb:277:in `each'
  from /home/deivid/Code/rubygems/lib/rubygems/resolver.rb:277:in `sort_by'
  from /home/deivid/Code/rubygems/lib/rubygems/resolver.rb:277:in `with_index'
  from /home/deivid/Code/rubygems/lib/rubygems/resolver.rb:277:in `sort_dependencies'
  from /home/deivid/Code/rubygems/lib/rubygems/resolver/molinillo/lib/molinillo/delegates/specification_provider.rb:52:in `block in sort_dependencies'
  from /home/deivid/Code/rubygems/lib/rubygems/resolver/molinillo/lib/molinillo/delegates/specification_provider.rb:69:in `with_no_such_dependency_error_handling'
  from /home/deivid/Code/rubygems/lib/rubygems/resolver/molinillo/lib/molinillo/delegates/specification_provider.rb:51:in `sort_dependencies'
  from /home/deivid/Code/rubygems/lib/rubygems/resolver/molinillo/lib/molinillo/resolution.rb:165:in `initial_state'
  from /home/deivid/Code/rubygems/lib/rubygems/resolver/molinillo/lib/molinillo/resolution.rb:106:in `start_resolution'
  from /home/deivid/Code/rubygems/lib/rubygems/resolver/molinillo/lib/molinillo/resolution.rb:64:in `resolve'
  from /home/deivid/Code/rubygems/lib/rubygems/resolver/molinillo/lib/molinillo/resolver.rb:42:in `resolve'
  from /home/deivid/Code/rubygems/lib/rubygems/resolver.rb:188:in `resolve'
  from /home/deivid/Code/rubygems/lib/rubygems/request_set.rb:396:in `resolve'
  from /home/deivid/Code/rubygems/lib/rubygems/request_set.rb:408:in `resolve_current'
  from /home/deivid/Code/rubygems/lib/rubygems.rb:243:in `finish_resolve'
  from /home/deivid/Code/rubygems/lib/rubygems/rdoc.rb:13:in `<top (required)>'
  from /home/deivid/Code/rubygems/lib/rubygems/core_ext/kernel_require.rb:54:in `require'
  from /home/deivid/Code/rubygems/lib/rubygems/core_ext/kernel_require.rb:54:in `require'
  from /home/deivid/Code/rubygems/lib/rubygems/test_case.rb:1563:in `<top (required)>'
  from /home/deivid/Code/rubygems/test/rubygems/test_bundled_ca.rb:2:in `require'
  from /home/deivid/Code/rubygems/test/rubygems/test_bundled_ca.rb:2:in `<top (required)>'
  from /home/deivid/.rbenv/versions/2.4.4/lib/ruby/gems/2.4.0/gems/rake-12.0.0/lib/rake/rake_test_loader.rb:15:in `require'
  from /home/deivid/.rbenv/versions/2.4.4/lib/ruby/gems/2.4.0/gems/rake-12.0.0/lib/rake/rake_test_loader.rb:15:in `block in <main>'
  from /home/deivid/.rbenv/versions/2.4.4/lib/ruby/gems/2.4.0/gems/rake-12.0.0/lib/rake/rake_test_loader.rb:4:in `select'
  from /home/deivid/.rbenv/versions/2.4.4/lib/ruby/gems/2.4.0/gems/rake-12.0.0/lib/rake/rake_test_loader.rb:4:in `<main>'
rake aborted!
Command failed with status (1)

Tasks: TOP => test
```

Now the explanation of the error:

* Rubygems base `TestCase` class requires `bundler` because some tests
use `bundler`:
2bbcdcde08/lib/rubygems/test_case.rb (L26)

* That `require` (our custom rubygems require) would activate the
default bundler spec (1.16.1 for ruby 2.4.4) but then overwrite it with
a 1.16.2 version (the locally provided bundler those days) due to [this
old
hack](9f7bf0ac3a/lib/bundler/version.rb (L7-L23)).

* Rubygems base `TestCase` class requires `rubygems/rdoc`:
2bbcdcde08/lib/rubygems/test_case.rb (L1536)

* And that file ends up calling `Gem.finish_resolve`:
2bbcdcde08/lib/rubygems/rdoc.rb (L13)

* `Gem.finish_resolve` adds the currently loaded specs to the
resolution:
2bbcdcde08/lib/rubygems.rb (L235)

* That means it would try to resolve bundler 1.16.2, but no
specification for that version was installed since the default was
1.16.1. That explains why upgrading to rubygems 2.7.7 fixed the issue,
since it provided bundler 1.16.2 by default so there was not bundler
version discrepancy.

After understanding the error, I conclude that:

* Only this part of the original patch was actually needed to resolve
the error, not any of the changes in `Gem::Specification.stubs` and
`Gem::Specification.stubs_for`:

```diff
diff --git a/lib/rubygems/test_case.rb b/lib/rubygems/test_case.rb
index f1cd3d274c..92c848e870 100644
--- a/lib/rubygems/test_case.rb
+++ b/lib/rubygems/test_case.rb
@@ -13,6 +13,15 @@ else
   require 'rubygems'
 end

+# If bundler gemspec exists, add to stubs
+bundler_gemspec = File.expand_path("../../../bundler/bundler.gemspec", __FILE__)
+if File.exist?(bundler_gemspec)
+  Gem::Specification.dirs.unshift File.dirname(bundler_gemspec)
+  Gem::Specification.class_variable_set :@@stubs, nil
+  Gem::Specification.stubs
+  Gem::Specification.dirs.shift
+end
+
 begin
   gem 'minitest'
 rescue Gem::LoadError
```

So, I propose to revert adding loaded specification to
`Gem::Specification.stubs` and `Gem::Specification.stubs_for` because I
think it's safe, it fixes the issues caused by their addition, and it
simplifies `Gem::Specification` code, which is already complicated
enough.

https://github.com/rubygems/rubygems/commit/5269cd617c

lib/rubygems/specification.rb

@@ -801,7 +801,7 @@ class Gem::Specification < Gem::BasicSpecification
   def self.stubs
     @@stubs ||= begin
       pattern = "*.gemspec"
-      stubs = Gem.loaded_specs.values + installed_stubs(dirs, pattern) + default_stubs(pattern)
+      stubs = installed_stubs(dirs, pattern) + default_stubs(pattern)
       stubs = stubs.uniq { |stub| stub.full_name }
 
       _resort!(stubs)
@@ -832,9 +832,7 @@ class Gem::Specification < Gem::BasicSpecification
       @@stubs_by_name[name]
     else
       pattern = "#{name}-*.gemspec"
-      stubs = Gem.loaded_specs.values +
-        installed_stubs(dirs, pattern).select { |s| Gem::Platform.match s.platform } +
-        default_stubs(pattern)
+      stubs = installed_stubs(dirs, pattern).select { |s| Gem::Platform.match s.platform } + default_stubs(pattern)
       stubs = stubs.uniq { |stub| stub.full_name }.group_by(&:name)
       stubs.each_value { |v| _resort!(v) }
 

test/rubygems/test_gem_specification.rb

@@ -1137,48 +1137,6 @@ dependencies: []
     refute_includes Gem::Specification.stubs.map { |s| s.full_name }, 'a-1'
   end
 
-  def test_self_stubs
-    Gem.loaded_specs.clear
-    Gem::Specification.class_variable_set(:@@stubs, nil)
-
-    dir_standard_specs = File.join Gem.dir, 'specifications'
-    dir_default_specs = Gem.default_specifications_dir
-
-    # Create gemspecs in three locations used in stubs
-    loaded_spec = Gem::Specification.new 'a', '3'
-    Gem.loaded_specs['a'] = loaded_spec
-    save_gemspec 'a', '2', dir_default_specs
-    save_gemspec 'a', '1', dir_standard_specs
-
-    full_names = ['a-3', 'a-2', 'a-1']
-    assert_equal full_names, Gem::Specification.stubs.map { |s| s.full_name }
-
-    Gem.loaded_specs.delete 'a'
-    Gem::Specification.class_variable_set(:@@stubs, nil)
-  end
-
-  def test_self_stubs_for
-    Gem.loaded_specs.clear
-    Gem::Specification.class_variable_set(:@@stubs, nil)
-
-    dir_standard_specs = File.join Gem.dir, 'specifications'
-    dir_default_specs = Gem.default_specifications_dir
-
-    # Create gemspecs in three locations used in stubs
-    loaded_spec = Gem::Specification.new 'a', '3'
-    Gem.loaded_specs['a'] = loaded_spec
-    save_gemspec('a-2', '2', dir_default_specs) { |s| s.name = 'a' }
-    save_gemspec('a-1', '1', dir_standard_specs) { |s| s.name = 'a' }
-
-    full_names = ['a-3', 'a-2', 'a-1']
-
-    assert_equal full_names, Gem::Specification.stubs_for('a').map { |s| s.full_name }
-    assert_equal 1, Gem::Specification.class_variable_get(:@@stubs_by_name).length
-
-    Gem.loaded_specs.delete 'a'
-    Gem::Specification.class_variable_set(:@@stubs, nil)
-  end
-
   def test_self_stubs_for_lazy_loading
     Gem.loaded_specs.clear
     Gem::Specification.class_variable_set(:@@stubs, nil)