closes#93.
note that this includes classes:
- `nixos`
- `nixosTest`
- `nixops4Resource`
- `nixops4Deployment`
.. and my (made-up, as per the [docs](https://ryantm.github.io/nixpkgs/module-system/module-system/#module-system-lib-evalModules-param-class)):
- `nix-unit`
- `package`
.. while i did not manage to cover:
- service tests, given `pkgs.nixosTest` seemed to not actually like `_class = "nixosTest"` (?!)
... nor #93's mentioned destructured arguments for that matter, as per Fediversity/Fediversity#93 (comment) - let me know if that is still desired as well.
Reviewed-on: Fediversity/Fediversity#398
Reviewed-by: Valentin Gagarin <valentin.gagarin@tweag.io>
Co-authored-by: Kiara Grouwstra <kiara@procolix.eu>
Co-committed-by: Kiara Grouwstra <kiara@procolix.eu>
