Enable `freeze` and `list` to introspect non-virtualenv Pythons (#2033)
## Summary
Now that we have the ability to introspect the installed packages for
arbitrary Pythons, we can allow `pip freeze` and `pip list` to fall back
to the "default" Python, if no virtualenv is present.
Closes https://github.com/astral-sh/uv/issues/2005.