GH-117586: Speed up `pathlib.Path.walk()` by working with strings (#117726)

Move `pathlib.Path.walk()` implementation into `glob._Globber`. The new
`glob._Globber.walk()` classmethod works with strings internally, which is
a little faster than generating `Path` objects and keeping them normalized.
The `pathlib.Path.walk()` method converts the strings back to path objects.

In the private pathlib ABCs, our existing subclass of `_Globber` ensures
that `PathBase` instances are used throughout.

Follow-up to #117589.