[rubygems/rubygems] Make the user installation less exceptional

The main purpose is to put handling of user installation into the same
place as e.g. handling the --build-root option handling. There is no
reason why the --build-root option should not prefix also paths used for
user installation.

Please note that the `util_installer` in
`test_generate_plugins_with_user_install` enforced the `:install_dir`,
which is against what user install is about.

https://github.com/rubygems/rubygems/commit/0b10cb41aa

lib/rubygems/installer.rb

@@ -189,12 +189,6 @@ class Gem::Installer
     @package.prog_mode = options[:prog_mode]
     @package.data_mode = options[:data_mode]
 
-    if options[:user_install]
-      @gem_home = Gem.user_dir
-      @bin_dir = Gem.bindir gem_home unless options[:bin_dir]
-      @plugins_dir = Gem.plugindir(gem_home)
-    end
-
     if @gem_home == Gem.user_dir
       # If we get here, then one of the following likely happened:
       # - `--user-install` was specified
@@ -673,22 +667,27 @@ class Gem::Installer
     @env_shebang         = options[:env_shebang]
     @force               = options[:force]
     @install_dir         = options[:install_dir]
-    @gem_home            = options[:install_dir] || Gem.dir
-    @plugins_dir         = Gem.plugindir(@gem_home)
     @ignore_dependencies = options[:ignore_dependencies]
     @format_executable   = options[:format_executable]
     @wrappers            = options[:wrappers]
     @only_install_dir    = options[:only_install_dir]
 
-    # If the user has asked for the gem to be installed in a directory that is
-    # the system gem directory, then use the system bin directory, else create
-    # (or use) a new bin dir under the gem_home.
-    @bin_dir             = options[:bin_dir] || Gem.bindir(gem_home)
+    @bin_dir             = options[:bin_dir]
     @development         = options[:development]
     @build_root          = options[:build_root]
 
     @build_args = options[:build_args]
 
+    @gem_home = @install_dir
+    @gem_home ||= options[:user_install] ? Gem.user_dir : Gem.dir
+
+    # If the user has asked for the gem to be installed in a directory that is
+    # the system gem directory, then use the system bin directory, else create
+    # (or use) a new bin dir under the gem_home.
+    @bin_dir ||= Gem.bindir(@gem_home)
+
+    @plugins_dir = Gem.plugindir(@gem_home)
+
     unless @build_root.nil?
       @bin_dir = File.join(@build_root, @bin_dir.gsub(/^[a-zA-Z]:/, ""))
       @gem_home = File.join(@build_root, @gem_home.gsub(/^[a-zA-Z]:/, ""))

test/rubygems/test_gem_installer.rb

@@ -821,7 +821,7 @@ gem 'other', version
     File.chmod(0o555, Gem.plugindir)
     system_path = File.join(Gem.plugindir, "a_plugin.rb")
     user_path = File.join(Gem.plugindir(Gem.user_dir), "a_plugin.rb")
-    installer = util_installer spec, Gem.dir, :user
+    installer = Gem::Installer.at spec.cache_file, :user_install => true, :force => true
 
     assert_equal spec, installer.install