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

@@ -114,13 +114,14 @@ let
       ''
 
         (module:
-        lib.evalModules {
+        lib.evalModules ({
           specialArgs = {
             inherit self flake-parts-lib moduleLocation;
             inputs = args.inputs or /* legacy, warned above */ self.inputs;
           } // specialArgs;
           modules = [ ./all-modules.nix (lib.setDefaultModuleLocation errorLocation module) ];
-        }
+          class = "flake";
+        })
         );
 
     # Function to extract the default flakeModule from

modules/perSystem.nix

@@ -120,13 +120,14 @@ in
         };
       });
       apply = modules: system:
-        (lib.evalModules {
+        (lib.evalModules ({
           inherit modules;
           prefix = [ "perSystem" system ];
           specialArgs = {
             inherit system;
           };
-        }).config;
+          class = "perSystem";
+        })).config;
     };
 
     allSystems = mkOption {