Add wheel finders and restore the patched egg finder.

pkg_resources/setuptools doesn't come with wheel finders the same way that
it comes with egg finders out of the box.  This establishes parity between
egg and whl support in pkg_resources.

This also fixes a nasty bug where resource_listdir/resource_stream failed to
work for eggs within .pex files -- we managed to skirt by most issues since
almost all eggs that we were reading resources from were marked as
not-zip-safe and we were falling back to the disk-based egg
metadata/resource provider.  See FixedEggMetadata and the corresponding
linked setuptools issue.

Reviewed at https://rbcommons.com/s/twitter/r/86/