GH-102613: Improve performance of `pathlib.Path.rglob()` (GH-104244)

Stop de-duplicating results in `_RecursiveWildcardSelector`. A new
`_DoubleRecursiveWildcardSelector` class is introduced which performs
de-duplication, but this is used _only_ for patterns with multiple
non-adjacent `**` segments, such as `path.glob('**/foo/**')`. By avoiding
the use of a set, `PurePath.__hash__()` is not called, and so paths do not
need to be stringified and case-normalised.

Also merge adjacent '**' segments in patterns.