refactor: use 'default.nix' for all modules

modules/printing/default.nix

@@ -0,0 +1,17 @@
+{ pkgs, ... }:
+
+{
+  # services.avahi = {
+  #   enable = true;
+  #   nssmdns4 = true;
+  #   openFirewall = true;
+  # };
+
+  services.printing = {
+    enable = true;
+    # drivers = with pkgs; [
+    #   cups-filters
+    #   cups-browsed
+    # ];
+  };
+}