refactor(flake): re-organise systems and modules

system/nixos/default.nix

@@ -0,0 +1,21 @@
+{ inputs, username }:
+
+let
+  configuration = import ./configuration.nix;
+  hardware-configuration = import ./hardware-configuration.nix;
+in
+inputs.nixpkgs.lib.nixosSystem {
+  modules = [
+    inputs.home-manager.nixosModules.home-manager {
+      home-manager = {
+        extraSpecialArgs = { inherit inputs; };
+        useGlobalPkgs = true;
+        useUserPackages = true;
+        users."${username}" = import ./home-manager.nix;
+      };
+    }
+
+    configuration
+    hardware-configuration
+  ];
+}