GH-116380: Move pathlib-specific code from `glob` to `pathlib._abc`. (#120011)

In `glob._Globber`, move pathlib-specific methods to `pathlib._abc.PathGlobber` and replace them with abstract methods. Rename `glob._Globber` to `glob._GlobberBase`. As a result, the `glob` module is no longer befouled by code that can only ever apply to pathlib.

No change of behaviour.