Improve 'pip index versions' output to explain why it does not some versions visible on PyPI

[uranusjr]

Follow-up work to #8978. Quoting: #8978 (review)

A general note: The current implementation lists only applicable versions of a project, but that behaviour (same as pip install etc.) sometimes leads to a common user confusion: Why does pip not see a newly published version?

I guess the current impleemntation is probably good enough as an initial effort, but eventually we probably need to also list “found but not applicable” versions as well (and potentially also why they are not applicable). I’m not sure how it can be done best, however.