refactor(wpcarro/nixos): Define shared nopn.nix module

Shared between `ava` and `tarasco`. Also define `lib/default.nix` to share
utility functions like `usermod` between NixOS configurations.

Change-Id: I65b8d37520426e164c9d5f722bee4731a3c6e641
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5942
Tested-by: BuildkiteCI
Reviewed-by: wpcarro <wpcarro@gmail.com>
Autosubmit: wpcarro <wpcarro@gmail.com>

users/wpcarro/nixos/ava/default.nix

@@ -3,6 +3,7 @@
 
 let
   inherit (depot.users) wpcarro;
+  inherit (depot.users.wpcarro.lib) usermod;
 
   wpcarrosEmacs = wpcarro.emacs.nixos {
     load = [ ./ava.el ];
@@ -15,7 +16,9 @@ let
   };
 in
 {
-  imports = [ ./hardware.nix ];
+  imports = [
+    (usermod "hardware/nopn.nix")
+  ];
 
   # Use the TVL binary cache
   tvl.cache.enable = true;