Add class: imports "type checking"

Initially this had some feature checking with lib.functionArgs, but
I don't think this is useful. The _class attribute is not supported
by long-unsupported Nixpkgs (<23.05) anyway, so let's keep it simple.

extras/flakeModules.nix

@@ -16,6 +16,7 @@ let
       _file = "${toString moduleLocation}#flakeModules.${k}";
       key = "${toString moduleLocation}#flakeModules.${k}";
       imports = [ v ];
+      _class = "flake";
     });
     description = ''
       flake-parts modules for use by other flakes.

lib.nix

@@ -120,6 +120,7 @@ let
             inputs = args.inputs or /* legacy, warned above */ self.inputs;
           } // specialArgs;
           modules = [ ./all-modules.nix (lib.setDefaultModuleLocation errorLocation module) ];
+          class = "flake";
         }
         );
 

modules/perSystem.nix

@@ -126,6 +126,7 @@ in
           specialArgs = {
             inherit system;
           };
+          class = "perSystem";
         }).config;
     };