le loader will be asked for the source only when getlines is
    called, not immediately.

    If there is an entry in the cache already, it is not altered.

    :return: True if a lazy load is registered in the cache,
        otherwise False. To register such a load a module loader with a
        get_source method must be found, the filename must be a cachable
        filename, and the filename must not be already cached.
    r