chore(nix): update package reference

shell.nix

@@ -1,12 +1,12 @@
 {
   callPackage,
   go,
-}: let
-  mainPkg = callPackage ./default.nix {};
+}:
+let
+  mainPkg = callPackage ./package.nix { };
 in
 mainPkg.overrideAttrs (oa: {
-    nativeBuildInputs =
-      [
+  nativeBuildInputs = [
     go
   ]
   ++ (oa.nativeBuildInputs or [ ]);