Adds a directory path as a source.

    Args:
      dirpath: a string representing a path to the directory.

    Returns:
      True if the directory is not an already existing source.

def AddDir(self, dirpath):
    """Adds a directory path as a source.

    Args:
      dirpath: a string representing a path to the directory.

    Returns:
      True if the directory is not an already existing source.
    """
    if dirpath not in self._dirs:
      self._dirs.add(dirpath)
      return True
    return False