Disallow pyproject.toml from pip uninstall

[charliermarsh]

Summary

Passing pyproject.toml or setup.py to pip uninstall is a bit strange, since it will often require running a resolution to resolve the dependencies (e.g., build the project), which means we also need to accept --index-url and friends.

[charliermarsh]

I'm open to allowing it for pip uninstall if there's pushback. It's more work and I'll need to wire up all the arguments, but it's doable.

[zanieb]

uninstall seems like a really weird use case..

sync seems reasonable if we performed resolution... seems better for a future interface though.

[charliermarsh]

I changed it to just pip uninstall for now, since that's what was blocking me elsewhere.