modules: use consistent service variables

Benefits of adding top-level variables for used services:
- Makes it obvious which other services are referenced by a service
- Less code

We already do this in many other places.

modules/clightning.nix

@@ -76,6 +76,7 @@ let
   cfg = config.services.clightning;
   nbLib = config.nix-bitcoin.lib;
   nbPkgs = config.nix-bitcoin.pkgs;
+
   network = config.services.bitcoind.makeNetworkName "bitcoin" "regtest";
   configFile = pkgs.writeText "config" ''
     network=${network}