[rubygems/rubygems] Fix yanked gems being unintentionally update when other gems are unlocked

This is a regression from a change intended to raise errors when user
puts a gem under an incorrect source in the Gemfile by mistake. To fix
the issue, we revert the change that caused it and implement it in a
different way that restores the resolver independency from real
specifications. Now it deals only with names and versions and does not
try to materialize anything into real specifications before resolving.

d2bf1b86eb

spec/bundler/resolver/basic_spec.rb

@@ -233,7 +233,7 @@ Bundler could not find compatible versions for gem "a":
     it "resolves foo only to latest patch - changing dependency declared case" do
       # bar is locked AND a declared dependency in the Gemfile, so it will not move, and therefore
       # foo can only move up to 1.4.4.
-      @base << Bundler::LazySpecification.new("bar", "2.0.3", nil)
+      @base << build_spec("bar", "2.0.3").first
       should_conservative_resolve_and_include :patch, ["foo"], %w[foo-1.4.4 bar-2.0.3]
     end