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.nix

@@ -167,9 +167,9 @@ in {
       }];
       version = 3;
     };
-    environment.systemPackages = with pkgs; [
+    environment.systemPackages = with pkgs; with nix-bitcoin; [
       tor
-      blockchains.bitcoind
+      bitcoind
       (hiPrio config.services.bitcoind.cli)
       nodeinfo
       jq