[pixi build] Add support for import-namespace and import-names in pyproject.toml [PEP-794]

[idantene]

Problem description

Hey!

PEP-794 introduced the notion of import name metadata, approved in Sept 2025.

I'm building several packages with a common namespace, and pixi fails on the pyproject-compliant specifications of package.import-namespace and package.import-names.
It would be great if pixi could support these out of the box instead :)

Thanks!

[ruben-arts]

Does it just fail to build due to a parsing error or are you missing implementation related items on the import-names feature in the pyproject.toml?

[idantene]

It fails because the schema rejects it (the keywords are not listed so they're not accepted).

I'm sure pixi could utilize this added information somehow, but I'm not expecting it (and can't think of a use-case right now), so it's just the schema AFAICS.