nanopos, lightning-charge and spark-wallet: Package via node2nix

The strategy of invoking node2nix inside a derivation (installPhase in
this case) does not work, as under NixOS installations there is no
network traffic allowed during a derivation build. Hence, we move
node2nix outside and rewrite the packaging into the modules.

Also switch to callPackage instead of plain imports. This could
probably be done on all other imported packages inside of
nix-bitcoin-pkgs.nix.

modules/nix-bitcoin.nix

@@ -14,9 +14,9 @@ let
   ];
   allPackages = with pkgs; [
     liquidd
-    lightning-charge.package
-    nanopos.package
-    spark-wallet.package
+    lightning-charge
+    nanopos
+    spark-wallet
     electrs
     nodejs-8_x
     nginx