def _get_required_fn(fn, root_path):
    """
    Definition of the MD5 file requires, that all paths will be absolute
    for the package directory, not for the filesystem.

    This function converts filesystem-absolute paths to package-absolute paths.

    Args:
        fn (str): Local/absolute path to the file.
        root_path (str): Local/absolute path to the package directory.

    Returns:
        str: Package-absolute path to the file.

    Raises:
        ValueError: When `fn` is absolute and `root_path` relative or                     conversely.
    """
    if not fn.startswith(root_path):
        raise ValueError('Both paths have to be absolute or local!')
    replacer = '/' if root_path.endswith('/') else ''
    return fn.replace(root_path, replacer, 1)
