pip check should be able to detect broken packages (missing files)

[ssbarnea]

I recently discovered that "pip check" is not able to detect broken installations where some files from the distributed package are missing.

This can happen quite easily with packages that do share same namespaces or that changed namespaces. A practical example is related to ansible and ansible-base.

Pip should check if all files from the dist are present and report an error, this may be slow so we may want to make this an optional feature, still very useful for validating the state of the packages present.

[webknjaz]

In particular, pip should be able to see when, as a result of a scenario described in https://github.com/pypa/pip/issues/8509, files that are listed in dist metadata are actually present on disk at the end of the install.

[fbidu]

Hello all!

At the EuroPython sprints today, I tried to come up with a failing test case for this at #8633

There's probably a lot to discuss about this (optional?) feature for check but I hope I can contribute somehow. It is my first time around pip codebase, so I maybe didn't do things in a very proper way. Feedback is always appreciated!

Thanks