[rubygems/rubygems] Metadata dependencies can be Gem::Dependency instances

They use less memory that way.

When resolving from scratch a Gemfile including only `"gem "rails", "~>
8.0.1"`, I get the following results:

### Before

Total allocated: 265.06 MB (3186053 objects)
Total retained:  116.98 MB (1302280 objects)

### After

Total allocated: 262.99 MB (3177437 objects)
Total retained:  115.91 MB (1297821 objects)

https://github.com/rubygems/rubygems/commit/a4ef9c5f56

lib/bundler/resolver/spec_group.rb

@@ -82,7 +82,7 @@ module Bundler
       def metadata_dependency(name, requirement)
         return if requirement.nil? || requirement.none?
 
-        Dependency.new("#{name}\0", requirement)
+        Gem::Dependency.new("#{name}\0", requirement)
       end
     end
   end