Formally added deprecation messages for ``namespace_packages``.
The methodology that uses ``pkg_resources`` and ``namespace_packages`` for
creating namespaces was already discouraged by the :doc:`setuptools docs
</userguide/package_discovery>` and the
:doc:`Python Packaging User Guide <PyPUG:guides/packaging-namespace-packages>`,
therefore this change just make the deprecation more official.
Users can consider migrating to native/implicit namespaces (as introduced in
:pep:`420`).
