move system packages declaration to modules dir

nixos/configuration.nix

@@ -15,6 +15,7 @@ in
       # Include the results of the hardware scan.
       ./hardware-configuration.nix
       ./packages.nix
+      ./modules/nitrokey.nix
     ];
 
   services.davfs2.enable = true;
@@ -143,8 +144,6 @@ in
     package = pkgs.pulseaudioFull;
   };
 
-  # Nitrokey support
-  services.udev.packages = [ pkgs.nitrokey-udev-rules ];
 
   # Enable the X11 windowing system.
   services.xserver.enable = true;