Respect `--with` versions over base environment versions (#14863)
## Summary
This fixes a regression from https://github.com/astral-sh/uv/pull/14447
that we seemingly didn't have test coverage for. Specifically, if you
have a version of a package in your project, and then install a
different version with `--with`, the environment should import the
`--with` version.
Closes #14860.
