## Summary
This PR modifies the resolver to treat the Python version as a package,
which allows for better error messages (since we no longer treat
incompatible packages as if they "don't exist at all").
There are a few tricky pieces here...
First, we need to track both the interpreter's Python version and the
_target_ Python version, because we support resolving for other versions
via `--python 3.7`.
Second, we allow using incompatible wheels during resolution, as long as
there's a compatible source distribution. So we still need to test for
`requires-python` compatibility when selecting distributions.
This could use more testing, but it feels like an area where `packse`
would be more productive than writing PyPI tests.
Closes https://github.com/astral-sh/puffin/issues/406.
Charlie Marsh