lib.modules: in evalModules return move _module.class -> configurationClass

doc/module-system/module-system.chapter.md

···
 A portion of the configuration tree which is elided from `config`.
 
 <!-- TODO: when markdown migration is complete, make _module docs visible again and reference _module docs. Maybe move those docs into this chapter? -->
+
+#### `_type` {#module-system-lib-evalModules-return-value-_type}
+
+A nominal type marker, always `"configuration"`.
+
+#### `configurationClass` {#module-system-lib-evalModules-return-value-_configurationClass}
+
+Equal to the [`class` parameter](#module-system-lib-evalModules-param-class).

lib/modules.nix

···
               within a configuration, but can be used in module imports.
             '';
           };
-
-          _module.class = mkOption {
-            readOnly = true;
-            internal = true;
-            description = lib.mdDoc ''
-              If the `class` attribute is set and non-`null`, the module system will reject `imports` with a different `class`.
-
-              This option contains the expected `class` attribute of the current module evaluation.
-            '';
-          };
         };
 
         config = {
···
             inherit extendModules;
             moduleType = type;
           };
-          _module.class = class;
           _module.specialArgs = specialArgs;
         };
       };
···
         config = checked (removeAttrs config [ "_module" ]);
         _module = checked (config._module);
         inherit extendModules type;
+        configurationClass = class;
       };
     in result;
 

lib/tests/modules.sh

···
 
 # Class checks, evalModules
 checkConfigOutput '^{ }$' config.ok.config ./class-check.nix
+checkConfigOutput '"nixos"' config.ok.configurationClass ./class-check.nix
 checkConfigError 'The module .*/module-class-is-darwin.nix was imported into nixos instead of darwin.' config.fail.config ./class-check.nix
 checkConfigError 'The module foo.nix#darwinModules.default was imported into nixos instead of darwin.' config.fail-anon.config ./class-check.nix