Avoid unused extras check in `pip install` for source trees (#2811)
## Summary
This was an oversight in the `-r pyproject.toml` refactor. We can't
enforce unused extras if we have a source tree. We made the correct
changes to `pip compile`, but not `pip install`. This PR just mirrors
those changes to `pip install`, and adds a few tests.
Closes https://github.com/astral-sh/uv/issues/2801.
