refine flake-parts interface (translators)

[DavHau]

Currently the translator field accepts any string, and also there is no documentation for the translator options which differ between the individual translators.

I think in order to have nice documentation for the translators, they each need to have their own options like:

{
  perSystem.dream2nix.inputs.<name>.projects.<name>.translator = "package-lock";
  perSystem.dream2nix.inputs.<name>.projects.<name>.translatorArgs.package-lock.nodejs = 18;
}

While touching the translators, we should probably also:

• move them to /src/module to follow the same schema like our other modules. Currently translators still reside in /src/subsystems/{subsystem}/translators
• after doing the same with builders and discoverers, this will allow us to get rid of our legacy module loading mechanism defined in src/modules/subsystem-loading.nix

[DavHau]

I started implementing this via https://github.com/nix-community/dream2nix/pull/425. So far I only moved the package-lock translator as an example