def matching_files(spec, path)
    return [] unless @lib_dirs[spec.object_id] # case no paths
    glob = File.join @lib_dirs[spec.object_id], "#{path}#{Gem.suffix_pattern}"
    Dir[glob].select { |f| File.file? f.untaint }
  end
#
Returns files matching +path+ in +spec+.
--
Some of the intermediate results are cached in @lib_dirs for speed.