nixos,nixpkgs: Add module classes

This allows modules that declare their class to be checked.
While that's not most user modules, frameworks can take advantage
of this by setting declaring the module class for their users.
That way, the mistake of importing a module into the wrong hierarchy
can be reported more clearly in some cases.

nixos/modules/misc/documentation.nix

@@ -39,6 +39,7 @@ let
             _module.check = false;
           } ] ++ docModules.eager;
           specialArgs = specialArgs // {
+            class = "nixos";
             pkgs = scrubDerivations "pkgs" pkgs;
             # allow access to arbitrary options for eager modules, eg for getting
             # option types from lazy modules