Search a Loader data structure for a filename.

def _search(mapping, filename):
    """Search a Loader data structure for a filename."""
    result = mapping.get(filename)
    if result is not None:
        return result
    name, ext = os.path.splitext(filename)
    result = mapping.get(ext)
    if result is not None:
        for pattern, result2 in result:
            if fnmatch(filename, pattern):
                return result2
    return None