def collection_dir(*files)
      return directory if files.empty?

      site.in_source_dir(container, relative_directory, *files)
    end
def collection_dir(*files)
      return directory if files.empty?

      site.in_source_dir(container, relative_directory, *files)
    end
The full path to the directory containing the collection, with
   optional subpaths.

 *files - (optional) any other path pieces relative to the
           directory to append to the path

 Returns a String containing th directory name where the collection
   is stored on the filesystem.