def keep_dirs
      site.keep_files.map { |file| parent_dirs(site.in_dest_dir(file)) }.flatten.to_set
    end
def keep_dirs
      site.keep_files.map { |file| parent_dirs(site.in_dest_dir(file)) }.flatten.to_set
    end
Private: The list of directories that need to be kept because they are
 parent directories of files specified in keep_files

 Returns a Set with the directory paths