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/modules.nix

@@ -27,5 +27,7 @@ in {
 
   disabledModules = [ "services/networking/bitcoind.nix" ];
 
-  nixpkgs.overlays = [ (self: super: allPackages super) ];
+  nixpkgs.overlays = [ (self: super: {
+    nix-bitcoin = allPackages super;
+  }) ];
 }