Make repository importable as NUR (including an overlay)

https://github.com/nix-community/NUR is a Nix community project that
aims to make out of tree derivations more easily discoverable and
accessible to Nix users. Converting the nix-bitcoin repo to conform to
that style is a minor change and enhances reusability of its
components. For instance, I could slap on the clightning module more
easily onto my existing bitcoin node without having to redeploy the
whole as nixops driven installation. Having the repo in NUR style
would make that easier.

modules/nix-bitcoin-pkgs.nix

@@ -1,35 +1,19 @@
 { config, pkgs, ... }:
- let
-   nixpkgs-pinned = import ../pkgs/nixpkgs-pinned.nix;
-   nixpkgs-unstable = import nixpkgs-pinned.nixpkgs-unstable { };
-
-   # Custom packages
-   nodeinfo = pkgs.callPackage ../pkgs/nodeinfo { };
-   banlist = pkgs.callPackage ../pkgs/banlist { };
-   lightning-charge = pkgs.callPackage ../pkgs/lightning-charge { };
-   nanopos = pkgs.callPackage ../pkgs/nanopos { };
-   spark-wallet = pkgs.callPackage ../pkgs/spark-wallet { };
-   electrs = pkgs.callPackage ../pkgs/electrs { };
-   liquidd = pkgs.callPackage ../pkgs/liquidd { };
+let
+  nixpkgs-pinned = import ../pkgs/nixpkgs-pinned.nix;
+  nixpkgs-unstable = import nixpkgs-pinned.nixpkgs-unstable { };
 in {
   disabledModules = [ "services/security/tor.nix" ];
   imports = [
     (nixpkgs-pinned.nixpkgs-unstable + "/nixos/modules/services/security/tor.nix")
   ];
 
+  nixpkgs.overlays = [ (import ../overlay.nix) ];
+
   nixpkgs.config.packageOverrides = pkgs: {
     # Use bitcoin and clightning from unstable
     bitcoin = nixpkgs-unstable.bitcoin.override { };
     altcoins.bitcoind = nixpkgs-unstable.altcoins.bitcoind.override { };
     clightning = nixpkgs-unstable.clightning.override { };
-
-    # Add custom packages
-    inherit nodeinfo;
-    inherit banlist;
-    inherit lightning-charge;
-    inherit nanopos;
-    inherit spark-wallet;
-    inherit electrs;
-    inherit liquidd;
   };
 }