Adds a file path as a source.

    Args:
      filepath: a string representing a path to the file.

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

def AddFile(self, filepath):
    """Adds a file path as a source.

    Args:
      filepath: a string representing a path to the file.

    Returns:
      True if the file is not an already existing source.
    """
    if filepath not in self._files:
      self._files.add(filepath)
      return True
    return False