Make _check_dist_requires_python() parallel _check_link_requires_python(), and add more complete tests

[cjerdonek]

This PR (1) moves check_dist_requires_python() to resolve.py where it used, (2) makes its implementation closely parallel index.py's _check_link_requires_python(), (3) adds an ignore_requires_python argument (like_check_link_requires_python()), and (4) tests it quite fully (including log messages, exception messages, and all branch logic). It also sets us up for passing py_version_info to the Resolver class, which will help with other issues.

This is a refactor in preparation for addressing issue #5369 ("pip download --python-version fails when downloaded package doesn't support the Python version running pip") and related issues.

[pradyunsg]

check_dist_requires_python may be used in other locations too (like for checking the highest version of pip that supports a given version of Python once support is dropped; that's a feature request marked "good first issue"). I don't think it should be moved into resolve.py or suffixed with an _.

I do like the other changes in this PR.

[cjerdonek]

check_dist_requires_python may be used in other locations too

Can that wait until the future change actually happens? Right now _check_dist_requires_python() is only being used in one place, and the log messages in the function are specific to the use inside resolve.py. We might find that _check_dist_requires_python() isn't appropriate for future uses. This parallels the situation for _check_link_requires_python(), where the log messages in that function are specific and tied to the use inside index.py.

[cjerdonek]

As a compromise, can I move just _get_requires_python() to the other location? I can see that function being useful elsewhere because it doesn't have special logging logic, etc, and has a narrower purpose.

[pradyunsg]

can I move just _get_requires_python() to the other location?

Sounds fair to me.

[cjerdonek]

Okay, thanks, @pradyunsg. I've updated the PR.