add modules.nix

Importing modules.nix enables the stand-alone use of the modules, without the
config presets of nix-bitcoin.nix.

modules/default.nix

@@ -1,4 +1,5 @@
 {
+  modules = ./modules.nix;
   bitcoind = ./bitcoind.nix;
   clightning = ./clightning.nix;
   default = ./default.nix;
@@ -7,7 +8,6 @@
   liquid = ./liquid.nix;
   nanopos = ./nanopos.nix;
   nix-bitcoin = ./nix-bitcoin.nix;
-  nix-bitcoin-pkgs = ./nix-bitcoin-pkgs.nix;
   nix-bitcoin-webindex = ./nix-bitcoin-webindex.nix;
   spark-wallet = ./spark-wallet.nix;
   recurring-donations = ./recurring-donations.nix;