pathlib ABCs: remove duplicate `realpath()` implementation. (#119178)

Add private `posixpath._realpath()` function, which is a generic version of `realpath()` that can be parameterised with string tokens (`sep`, `curdir`, `pardir`) and query functions (`getcwd`, `lstat`, `readlink`). Also add support for limiting the number of symlink traversals.

In the private `pathlib._abc.PathBase` class, call `posixpath._realpath()` and remove our re-implementation of the same algorithm.

No change to any public APIs, either in `posixpath` or `pathlib`.

Co-authored-by: Nice Zombies <nineteendo19d0@gmail.com>