Refactor the whole configuration

Reviewed-on: #1

modules/nixos/core/nix.nix

@@ -0,0 +1,28 @@
+{ pkgs, lib, inputs, cfgPath, ... }:
+
+let
+  importOverlays = builtins.map
+    (overlay: import overlay { inherit lib inputs pkgs cfgPath; })
+    (builtins.filter 
+      (file: builtins.match ".*\.nix" (builtins.toString file) != null)
+      (lib.filesystem.listFilesRecursive "${cfgPath}/overlays"));
+in {
+  nix = {
+    settings = {
+      auto-optimise-store = true;
+      use-xdg-base-directories = true;
+      trusted-users = [ "@wheel" ];
+      allowed-users = [ "@wheel" ];
+    };
+    extraOptions = ''
+      experimental-features = nix-command flakes
+    '';
+  };
+  nixpkgs = {
+    config = {
+      allowUnfree = true;
+    };
+    hostPlatform = lib.mkDefault "x86_64-linux";
+    overlays = importOverlays;
+  };
+}