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/bitcoind.nix

@@ -69,7 +69,7 @@ in {
 
       package = mkOption {
         type = types.package;
-        default = pkgs.blockchains.bitcoind;
+        default = pkgs.nix-bitcoin.bitcoind;
         defaultText = "pkgs.blockchains.bitcoind";
         description = "The package providing bitcoin binaries.";
       };