lib.evalModules: Add extendModules and type to result

Allows the simultaneous construction of top-level invocations and
submodule types.

This helps structure configuration systems integration code.

lib/tests/modules/declare-submodule-via-evalModules.nix

@@ -0,0 +1,28 @@
+{ lib, ... }: {
+  options.submodule = lib.mkOption {
+    inherit (lib.evalModules {
+      modules = [
+        {
+          options.inner = lib.mkOption {
+            type = lib.types.bool;
+            default = false;
+          };
+        }
+      ];
+    }) type;
+    default = {};
+  };
+
+  config.submodule = lib.mkMerge [
+    ({ lib, ... }: {
+      options.outer = lib.mkOption {
+        type = lib.types.bool;
+        default = false;
+      };
+    })
+    {
+      inner = true;
+      outer = true;
+    }
+  ];
+}