GH-125413: Move `pathlib.Path.copy()` implementation alongside `Path.info` (#129856)

Move pathlib's private `CopyReader`, `LocalCopyReader`, `CopyWriter` and
`LocalCopyWriter` classes into `pathlib._os`, where they can live alongside
the low-level copying functions (`copyfileobj()` etc) and high-level path
querying interface (`PathInfo`).

This sets the stage for merging `LocalCopyReader` into `PathInfo`.

No change of behaviour; just moving some code around.