lib/modules: Only interpret class declaration in non-shorthand mode

This is to avoid stealing keys from submodules. `class` might be
common enough that reinterpreting existing `class` attributes in
configurations as a declaration leads to fairly widespread problems.

lib/modules.nix

···
         imports = m.require or [] ++ m.imports or [];
         options = {};
         config = addFreeformType (removeAttrs m ["_file" "key" "disabledModules" "require" "imports" "freeformType"]);
-        class = m.class or null;
+        class = null;
       };
 
   applyModuleArgsIfFunction = key: f: args@{ config, options, lib, ... }:

lib/tests/modules.sh

···
 ## Freeform modules
 # Assigning without a declared option should work
 checkConfigOutput '^"24"$' config.value ./freeform-attrsOf.nix ./define-value-string.nix
+# Shorthand modules interpret `meta` and `class` as config items
+checkConfigOutput '^true$' options._module.args.value.result ./freeform-attrsOf.nix ./define-freeform-keywords-shorthand.nix
 # No freeform assignments shouldn't make it error
 checkConfigOutput '^{ }$' config ./freeform-attrsOf.nix
 # but only if the type matches

lib/tests/modules/class-check.nix

···
           ./module-class-is-nixos.nix
           { _file = "foo.nix#darwinModules.default";
             class = "darwin";
+            config = {};
             imports = [];
           }
         ];

lib/tests/modules/define-freeform-keywords-shorthand.nix

···
+{ config, ... }: {
+  class = { "just" = "data"; };
+  a = "one";
+  b = "two";
+  meta = "meta";
+
+  _module.args.result =
+    let r = builtins.removeAttrs config [ "_module" ];
+    in builtins.trace (builtins.deepSeq r r) (r == {
+      a = "one";
+      b = "two";
+      class = { "just" = "data"; };
+      meta = "meta";
+    });
+}