Add routine for loading metadata from a source package.

I've created this MR to demonstrate how one could use pep517 and importlib.metadata to load metadata for an unbuilt package.

As currently implemented, it adds a Distribution.find_local(source_dir) method that only works if pep517 is installed (lazily imported).

Perhaps this functionality would be more suitably implemented in a submodule of packaging or maybe pep517 itself.