Add a Path-like API for filetree navigation

[jwodder]

Wheel/dist-info objects should have a filetrees (name WIP) attribute that is a dynamic mapping. Accessing this mapping with the keys "purelib", "platlib", "dist-info", "headers", "scripts", or "data" should return a PosixPath-like object for navigating through the respective subset of the files in the wheel. However — and this is key — it should not be possible to use such a path instance to access or view files outside the given filetree; this includes not being able to access the *.dist-info or *.data directories from the purelib tree (assuming root is purelib; otherwise, platlib).

• The filetrees mapping should accept any name not containing a slash; unknown names are treated as subdirectories of *.data. If a given directory does not exist (or if the corresponding entry in *.data is a file instead of a directory), None is returned.
• filetrees should accept a "root" (or None?) key to access purelib for purelib wheels and platlib for platlib wheels.
• These path objects should be accepted by the API wherever wheel entry filepaths are accepted.
• Path objects should have a wheel_path(?) attribute for getting the actual, full path of the underlying file inside the wheel.