Allow project.dependencies to be interpreted as conda-forge dependencies

[maresb]

Duplicate of #532

Problem description

Original Discord discussion

Currently pixi interprets project.dependencies as tool.pixi.pypi-dependencies. (At least I think so, but I'm not sure of the details. Does it also read from project.dependencies when using pixi.toml?) In order to install these dependencies instead with conda, they currently need to be duplicated to tool.pixi.dependencies.

My request is to add pixi.toml settings that accomplish the following:

1. Have a setting so that everything in project.dependencies is by default interpreted as a conda-forge dependency instead of a PyPI dependency
2. Be able to make an exception for individual packages, i.e. everything under project.dependencies should be a conda-forge dependency except for my-special-package-1 and my-special-package-2.
3. Be able to override the default pypi→conda-forge mapping in case it's wrong

Advanced considerations:

* Some people will want to use channels other than conda-forge (and pip repositories other than PyPI), and these might require authentication
* How to deal with multiple features/environments

[olivier-lacroix]

I believe this is a duplicate of #532

It’s something I want as well. I will implement it (but no promise on the timeline :-)

[tdejager]

@olivier-lacroix @maresb made it on my recommendenation. Forgot about the other issue, sorry about that! Should I merge this note at the end of that issue, as I think it has some good points laid out?

[maresb]

I'll migrate this to a comment on that other issue...

[tdejager]

Thanks @maresb, sorry about that!