add nix-bitcoin pkgs namespace

Not polluting the main pkgs namespace with internal pkgs makes it
easier to integrate the nix-bitcoin modules into a larger config.

Also, by overriding the nix-bitcoin namespace, users can now easily set the
packages used by services that offer no explicit `package` option, like `clightning`.

modules/nix-bitcoin-webindex.nix

@@ -74,7 +74,7 @@ in {
       description = "Get node info";
       wantedBy = [ "multi-user.target" ];
       after = [ "nodeinfo.service" ];
-      path  = [ pkgs.nodeinfo pkgs.clightning pkgs.jq pkgs.sudo ];
+      path  = with pkgs; [ nix-bitcoin.nodeinfo nix-bitcoin.clightning jq sudo ];
       serviceConfig = {
         ExecStart="${pkgs.bash}/bin/bash ${createWebIndex}";
         User = "root";