make default.nix a function callable by callPackage

build.nix

@@ -0,0 +1 @@
+with import <nixpkgs> {} ; callPackage ./. { }

default.nix

@@ -1,4 +1,9 @@
-with import <nixpkgs> {};
+{
+  lua5_3
+, stdenv
+, fennel
+, makeWrapper
+}:
 let lua = lua5_3.withPackages (ps: [
       ps.dkjson
       ps.lpeg
@@ -6,7 +11,7 @@ let lua = lua5_3.withPackages (ps: [
       ps.luaposix
     ]);
     inherit makeWrapper;
-    fennel = pkgs.fennel.override { inherit lua; };
+    myfennel = fennel.override { inherit lua; };
 in stdenv.mkDerivation {
   pname = "grafana-sms-alert";
   version = "0.1";
@@ -16,9 +21,9 @@ in stdenv.mkDerivation {
      lua_path="`lua -e 'print(package.path)'`"
      lua_cpath="`lua -e 'print(package.cpath)'`"
 
-     makeWrapper ${fennel}/bin/fennel $out/bin/grafana-sms-alert \
+     makeWrapper ${myfennel}/bin/fennel $out/bin/grafana-sms-alert \
       --add-flags "$out/lib/main.fnl"
   '';
-  buildInputs = [lua fennel];
+  buildInputs = [lua myfennel];
   nativeBuildInputs = [ makeWrapper ];
 }