lib/modules: Remove internal _module attribute from config

The _module option is added as an internal option set, and it messes up
the results of module evaluations, requiring people to manually filter
_modules out.

If people depend on this, they can still use config._module from inside
the modules, exposing _module as an explicitly declared user option. Or
alternatively with the _module attribute now returned by evalModules.

lib/modules.nix

@@ -93,7 +93,11 @@ rec {
             res set._definedNames
         else
           res;
-      result = { inherit options config; };
+      result = {
+        inherit options;
+        config = removeAttrs config [ "_module" ];
+        inherit (config) _module;
+      };
     in result;
 
   # collectModules :: (modulesPath: String) -> (modules: [ Module ]) -> (args: Attrs) -> [ Module ]