lib.modules: Let module declare options directly in bare submodule

... where a bare submodule is an option that has a type like
`submoduleWith x`, as opposed to `attrsOf (submoduleWith x)`.

This makes migration unnecessary when introducing a freeform type
in an existing option tree.

Closes #146882

lib/tests/modules/declare-bare-submodule-nested-option.nix

@@ -0,0 +1,18 @@
+{ lib, ... }:
+let
+  inherit (lib) mkOption types;
+in
+{
+  options.bare-submodule = mkOption {
+    type = types.submoduleWith {
+      modules = [
+        {
+          options.nested = mkOption {
+            type = types.int;
+            default = 1;
+          };
+        }
+      ];
+    };
+  };
+}