GH-106747: Make pathlib ABC globbing more consistent with `glob.glob()` (#115056)

When expanding `**` wildcards, ensure we add a trailing slash to the
topmost directory path. This matches `glob.glob()` behaviour:

    >>> glob.glob('dirA/**', recursive=True)
    ['dirA/', 'dirA/dirB', 'dirA/dirB/dirC']

This does not affect `pathlib.Path.glob()`, because trailing slashes aren't
supported in pathlib proper.