refactor: Remove 'defaults' abstraction

modules/nixos/nixpkgs.nix

@@ -0,0 +1,14 @@
+{ config, lib, pkgs, ... }:
+
+let cfg = config.sydnix.nixpkgs;
+in {
+  options.sydnix.nixpkgs = {
+    enable = lib.mkEnableOption "some default Nixpkgs settings" // {
+      default = true;
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    nixpkgs.config.allowUnfree = true;
+  };
+}