feat: Make project table optional for pyproject.toml manifests

[olivier-lacroix]

This Fixes #1687 and #1207 by keeping the [project] table optional in pyproject.toml.

The [project] table had been made mandatory to make sure a name was defined. Now, the project name is read from, in order of priority

• the [tool.pixi.project] table
• the [project] table
• the [tool.poetry] table

The same is done for project description, version and authors.

During init, if the name cannot be read from the [project] table or the [tool.poetry] table, a default name (inferred from the directory name) is added to the [tool.pixi.project]

This PR also refactors handling of pyproject.toml: both pixi and non-pixi pyproject.toml are now read into the same PyProjectManifest struct, with existence checking of the [tool.pixi.project] table managed outside of serde when necessary.

[ruben-arts]

Thanks !! Looks good to me! And works like a charm.

[ruben-arts]

Asked @tdejager for another quick review.

[tdejager]

LGTM! The only thing I would change, is maybe if the name does not exist add it to the pyproject project table instead of the tool.pixi this makes it more standard-compliant. The only downside I can see is if you want to not use that name for a wheel build, but that would be pretty strange. WDYT @olivier-lacroix ?

[tdejager]

With some closer look, it seems to be a bit difficult to do this, I would like it but lets postpone, I'll approve and merge as is for now.

[olivier-lacroix]

Thx @tdejager ! Maybe useful to have @pavelzw thoughts on this as well: I think I remember him not liking pixi imposing the creation of that table.

I do not see any difficulty in implementing it though: if project.name does not exist, this means there is no project table at all (as the field is mandatory), so we can 'just' insert it before the [tool.pixi] section. Or am I missing something?