feat: Implement more functions for pixi add --pypi

[wolfv]

This pr adds:

pixi add --pypi "boltons @ https://files.pythonhosted.org/packages/46/35/e50d4a115f93e2a3fbf52438435bb2efcf14c11d4fcd6bdcd77a6fc399c9/boltons-24.0.0-py3-none-any.whl"
pixi add --pypi "exchangelib @ git+https://github.com/ecederstrand/exchangelib"
pixi add --pypi "project @ file:///absolute/path/to/project"
pixi add --pypi --editable "project @ file:///absolute/path/to/project"

TODO:

• Edit: I don't see a way to do this.... Improve UX of the path style. (now seems to need absolute path, but this is not pixi like)

[wolfv]

thanks for taking this over! :)