mkvenv: replace distlib.database with importlib.metadata/pkg_resources

mkvenv: replace distlib.database with importlib.metadata/pkg_resources

importlib.metadata is just as good as distlib.database and a bit more
battle-proven for "egg" based distributions, and in fact that is exactly
why mkvenv.py is not using distlib.database to find entry points: it
simply does not work for eggs.

The only disadvantage of importlib.metadata is that it is not available
by default before Python 3.8, so we need a fallback to pkg_resources
(again, just like for the case of finding entry points).  Do so to
fix issues where incorrect egg metadata results in a JSONDecodeError.

While at it, reuse the new _get_version function to diagnose an incorrect
version of the package even if importlib.metadata is not available.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>