WIP introduce nixos module system for configuration

tests/module/defs.nix

@@ -0,0 +1,16 @@
+{ lib, ...}:
+let inherit (lib) mkEnableOption mkOption types;
+in {
+  options = {
+    services.a = {
+      enable = mkEnableOption "hello service";
+    };
+    services.b = {
+      enable = mkEnableOption "other service";
+    };
+    services.z = mkOption {   };
+    systemPackages = mkOption {
+      type = types.listOf types.package;
+    };
+  };
+}