GH-128520: Divide pathlib ABCs into three classes (#128523)

In the private pathlib ABCs, rename `PurePathBase` to `JoinablePath`, and
split `PathBase` into `ReadablePath` and `WritablePath`. This improves the
API fit for read-only virtual filesystems.

The split of `PathBase` entails a similar split of `CopyWorker` (implements
copying) and the test cases in `test_pathlib_abc`.

In a later patch, we'll make `WritablePath` inherit directly from
`JoinablePath` rather than `ReadablePath`. For a couple of reasons,
this isn't quite possible yet.