lib/modules: specify modulesPath

`specialArgs.modulesPath` can be used to allow users to manually
import modules relative to our `./modules` directory.

It is also used by the module system to provide a base path for
relative `disabledModules` paths.

lib/modules.nix

@@ -36,6 +36,7 @@ in
         }
       ];
       specialArgs = {
+        modulesPath = ../modules;
         # TODO: deprecate `helpers`
         helpers = self;
       } // extraSpecialArgs;