remove "parts" directory and restructure modules

modules/nix-defaults.nix

@@ -0,0 +1,19 @@
+{ pkgs, ... }:
+{
+  # for flakes
+  environment.systemPackages = [ pkgs.git ];
+
+  nix.settings = {
+    auto-optimise-store = true;
+    builders-use-substitutes = true;
+    experimental-features = [
+      "nix-command"
+      "flakes"
+    ];
+
+    trusted-users = [
+      "root"
+      "@wheel"
+    ];
+  };
+}