[rubygems/rubygems] Don't use full indexes unnecessarily on legacy Gemfiles

On legacy Gemfiles with multiple remote sources, where all of them
support the compact index API, we were still falling back to full
indexes.

Fixing this also allows to simplifying the code.

https://github.com/rubygems/rubygems/commit/b1357c8e72

lib/bundler/source/rubygems.rb

@@ -399,16 +399,11 @@ module Bundler
         @remote_specs ||= Index.build do |idx|
           index_fetchers = fetchers - api_fetchers
 
-          # gather lists from non-api sites
-          fetch_names(index_fetchers, nil, idx, false)
-
-          # legacy multi-remote sources need special logic to figure out
-          # dependency names and that logic can be very costly if one remote
-          # uses the dependency API but others don't. So use full indexes
-          # consistently in that particular case.
-          allow_api = !multiple_remotes?
-
-          fetch_names(api_fetchers, allow_api && dependency_names, idx, false)
+          if index_fetchers.empty?
+            fetch_names(api_fetchers, dependency_names, idx, false)
+          else
+            fetch_names(fetchers, nil, idx, false)
+          end
         end
       end
 

spec/bundler/install/gemfile/sources_spec.rb

@@ -35,6 +35,15 @@ RSpec.describe "bundle install with gems on multiple sources" do
         expect(the_bundle).to include_gems("rack-obama 1.0.0", "rack 1.0.0", :source => "remote1")
       end
 
+      it "does not use the full index unnecessarily", :bundler => "< 3" do
+        bundle :install, :artifice => "compact_index", :verbose => true
+
+        expect(out).to include("https://gem.repo1/versions")
+        expect(out).to include("https://gem.repo3/versions")
+        expect(out).not_to include("https://gem.repo1/quick/Marshal.4.8/")
+        expect(out).not_to include("https://gem.repo3/quick/Marshal.4.8/")
+      end
+
       it "fails", :bundler => "3" do
         bundle :install, :artifice => "compact_index", :raise_on_error => false
         expect(err).to include("Each source after the first must include a block")