Add test illustrating integration with packaging for installed version checks

@warsaw I'm particularly interested in your review of this change. It's simply the addition of a new test, but it illustrates a few interesting concerns:

• It adds a dependency on packaging (for tests only, but also for any callers that wish to implement this behavior).
• This test can't be ported to CPython, at least not without bundling packaging or otherwise making it available for the CPython tests.
• This change draws me a bit closer to dstufft's opinion that the functionality of importlib_metadata should be in packaging. I'd like to consider that concern again in a separate thread.