[rubygems/rubygems] Materialize specs just once in installer

`Definition#specs` does the same thing but memoizes the result.

ruby/rubygems@b62bf9fe41

Author: deivid-rodriguez

lib/bundler/installer.rb

@@ -213,7 +213,7 @@ def installation_parallelization
     def load_plugins
       Gem.load_plugins
 
-      requested_path_gems = @definition.requested_specs.select {|s| s.source.is_a?(Source::Path) }
+      requested_path_gems = @definition.specs.select {|s| s.source.is_a?(Source::Path) }
       path_plugin_files = requested_path_gems.flat_map do |spec|
         spec.matches_for_glob("rubygems_plugin#{Bundler.rubygems.suffix_pattern}")
       rescue TypeError