feat: dont remove non pixi pypi installs

[tdejager]

Check the INSTALLER when removing packages either through changing of dependencies or python version.

Fixes: #998

We now have the following behavior:

• If the installer is not uv-pixi and not managed by pixi we do not touch the package.
• If the installer is not uv-pixi but is managed by pixi we re-install.

Also, In the case where we switch python interpreters, which requires a new site-packages folder:

• We leave the packages alone that do not have our installer

---

The problem is that for existing installs these have the wrong installer, which is all installs done with an older pixi version: when this PR is merged the installer will automatically be updated and the following warning is shown once:

E.g for an environment with python and flask from pypi:

 INFO pixi::install_pypi: These pypi-packages were re-installed because they were previously installed by a different installer but are currently managed by pixi:
        markupsafe, blinker, click, flask, itsdangerous, jinja2, werkzeug

Because we are essentially re-installing all pypi-packages in the environment.

This is a bit ugly, but I've noticed people being suprised that if they tinker in an environment a re-install occurs.

[tdejager]

Before merging we need to re-think the logic a bit.

1. When we want to install a package that was not managed by us, but is managed by us now, we should still re-install.
2. Maybe add a warning when we find pypi packages that were not installed by us or something? So that people know that we've not touched those?

[wolfv]

Two points:

• when a package is installed (even by another package manager), but we want to install a different version of it, we should still remove it and install the pixi managed version (maybe with a warning?)
• when users have an existing environment and update it might look ugly for the transition period. Maybe for one or two pixi releases, we should keep the uv handling as well?

[tadejsv]

Any progress on this? @wolfv @tdejager

[tdejager]

Not yet, we should revive this. Although you can now use git dependencies if you want 😀

[abey79]

Awesome!