make support for `pixi add --pypi` source dependencies.

[ruben-arts]

Problem description

It would be nice for the users to add source dependencies from the command line. Like pip and uv do.

e.g.:

pixi add --pypi "SomeProject@git+https://git.repo/[email protected]"
pixi add --pypi  --editable .  
pixi add --pypi path/to/project --editable  

This needs some further ideas!

[tdejager]

pixi add --pypi -e . should be a no-op when using a pyproject.toml and we merge https://github.com/prefix-dev/pixi/pull/1084 because this will be done by default.

[ruben-arts]

Initial work is done some left over improvements that we would really like to support as well:

# Relative paths
pixi add --pypi "bla @ ./../../path/to/package"
pixi add --pypi "bla @ ./../../path/to/package.whl"

# Git Tags/Branches
pixi add --pypi "bla @ git+https://github.com/prefix-dev/[email protected]"
pixi add --pypi "bla @ git+https://github.com/prefix-dev/pixi-py@develop"

# Nameless requirements
pixi add --pypi ./path/to/package
pixi add --pypi .
pixi add --pypi path/to/wheel.whl
pixi add --pypi https://github.com/prefix-dev/pixi-py@develop