GH-127807: pathlib ABCs: move private copying methods to dedicated class (#127810)

Move 9 private `PathBase` attributes and methods into a new `CopyWorker`
class. Change `PathBase.copy` from a method to a `CopyWorker` instance.

The methods remain private in the `CopyWorker` class. In future we might
make some/all of them public so that user subclasses of `PathBase` can
customize the copying process (in particular reading/writing of metadata,)
but we'd need to make `PathBase` public first.