Always use base Python discovery logic for cached environments

[charliermarsh]

Summary

This is attempting to solve the same problem surfaced in #11208 and #11209. However, those PRs only worked for our own managed Pythons. In Gentoo, for example, they disable the managed Pythons, which led to failures in the test suite, because the "base Python" returned after creating a virtual environment would differ from the "base Python" that you get after querying an existing virtual environment.

The fix here is to apply our same base Python normalization and discovery logic, to non-standalone / non-managed Pythons. We continue to use sys._base_executable for such Pythons when creating the virtualenv, but when caching, we perform this second discovery step.

Closes #11237.

[charliermarsh]

\cc @mgorny

[zanieb]

Some commentary on how find_base_python and to_base_python differ would be nice. It's not obvious.

[charliermarsh]

There's a note in the rustdoc for each, but I'll expand it!

[zanieb]

Yeah I read that note, but am looking for a reference to the other / an explanation of when you would want one vs the other.