[rubygems/rubygems] Extract `SourceList#get_with_fallback`

https://github.com/rubygems/rubygems/commit/9dbc4757a8

lib/bundler/definition.rb

@@ -743,7 +743,7 @@ module Bundler
         # Replace the locked dependency's source with the equivalent source from the Gemfile
         dep = @dependencies.find {|d| s.satisfies?(d) }
 
-        s.source = (dep && dep.source) || sources.get(s.source) || sources.default_source
+        s.source = (dep && dep.source) || sources.get_with_fallback(s.source)
 
         next if @unlock[:sources].include?(s.source.name)
 

lib/bundler/source_list.rb

@@ -101,6 +101,10 @@ module Bundler
       source_list_for(source).find {|s| equivalent_source?(source, s) }
     end
 
+    def get_with_fallback(source)
+      get(source) || default_source
+    end
+
     def lock_sources
       lock_other_sources + lock_rubygems_sources
     end