Investigate possibility of including modules exposed by packages in PyPI

[mknorps]

We opened a PyPI issue to include modules exposed by packages as metadata in the PyPI public API.

PyPI developers reported difficulties in package introspection and the top_level.txt file may not be the only place where exposed modules are kept. This should be investigated.

The information on exposed modules could also be interesting to automatic requirements generation libraries, like pigar. We should contact them to check if such an improvement in PyPI is of interest to them as well.