Set `_file` and `_class` for modules defined under `flake.modules.${className}.${moduleName}`

[Atry]

In NixOS/nixpkgs#197547

I do not expect users or module authors to set the class attribute in their modules (although they may). Instead I envision a library like flake-parts to set this for us.

Since NixOS/nixpkgs#197547 is merged, do we want to introduce a module to set _file and _class for modules defined flake.modules.${className}.${moduleName}?

[roberth]

Yes, that's the plan.

• Similar to No access to flake inputs within a perSystem module #101 (comment)

Just haven't executed on it yet.
For a modules output, that should probably be in a separately imported module until nix flake check ignores that attribute. We're trying to move it away from such hardcoded logic in Nix, so it's highly unlikely that it will be checked within the current architecture, and that's ok. Maybe the default flake schema could do it, although I have some issues with that feature's design as currently implemented in a PR.