cleaned up configuration structure to have common modules folder

modules/default.nix

@@ -0,0 +1,7 @@
+# this folder is for modules that are common between nixos, home-manager, and darwin
+{...}: {
+  imports = [
+    ./overlays
+    ./pkgs
+  ];
+}

modules/overlays/default.nix

@@ -0,0 +1,3 @@
+# this folder is for derivation overlays
+{...}: {
+}

modules/pkgs/default.nix

@@ -0,0 +1,4 @@
+# this folder is for custom derivations
+{...}: {
+  # package = pkgs.callPackage ./package.nix {};
+}