refactor: delay channel config url evaluation

[baszalmstra]

Uses the NamedChannelOrUrl type from rattler to delay channel url evaluation.

[ruben-arts]

fixed the tests in baszalmstra#3

[wolfv]

This is great. Does this also mean we get relative file paths channels as relative to the pixi.toml now? :)

[baszalmstra]

Thanks @ruben-arts !

Im not quite done though. There are more uses that I wanna take care of.

[ruben-arts]

Great! Will put it on draft for the overview.

[wolfv]

For reference - the issue about relative channel paths: #1159

[baszalmstra]

For reference - the issue about relative channel paths: #1159

I added support for setting the proper root dir but Im not entirely sure if it works because parsing the NamelessMatchSpec also requires the root config.

But Im pretty sure that it does fix the issue referenced in the issue.

[ruben-arts]

Looks good to me