GH-125413: pathlib ABCs: use `scandir()` to speed up `walk()` (#126262)

Use the new `PathBase.scandir()` method in `PathBase.walk()`, which greatly
reduces the number of `PathBase.stat()` calls needed when walking.

There are no user-facing changes, because the pathlib ABCs are still
private and `Path.walk()` doesn't use the implementation in its superclass.