Rename nixosModules to modules.{nixos,darwin,arion,...}

[roberth]

Is your feature request related to a problem? Please describe.

NixOS isn't the only consumer of the module system. nix flake check (which I think is the only command that cares about modules anyway) should embrace this fact and we can exploit this in the flake schema.

Describe the solution you'd like

I propose to add an attribute that covers all module system applications:

nixosModules.* -> modules.nixos.*
darwinModules.* -> modules.nix-darwin.*
flakeModules.* -> modules.flake-parts.*
etc

This way, nix doesn't need to know about which module system applications exist, but can check the types of these values nonetheless, because all of modules.<application>.<moduleName> must be modules.

Describe alternatives you've considered

Other module system applications stay second-class, produce warnings and remain unchecked.

Ignore the pattern and throw everything into lib, which actually still produces a warning anyway #flakelib

Additional context

• DavHau/drv-parts@cc4cf9f#r91615629

• Policy for allowing flake outputs in nix flake check #6381

• tangentially, lib could be a supported output in flakes #4744

[bew]

Duplicate of #6257 ?
And #6723

[thufschmitt]

Duplicate of #6257 ? And #6723

Indeed. I'll close this in favour of #6257 to keep the discussion scoped.