move gitSource into dragnpkgs

package.nix

@@ -1,6 +1,7 @@
 {
   lib,
   mkShell,
+  gitSource,
 
   buildDunePackage,
   ppxlib,
@@ -14,17 +15,10 @@
   enableShell ? false
 }:
 
-let
-  sourceRoot = ./.;
-  fs = lib.fileset;
-  sourceFiles = fs.difference
-    (fs.gitTracked sourceRoot)
-    (fs.fileFilter (file: file.hasExt "nix") sourceRoot);
-  sourceSet = fs.toSource { root = sourceRoot; fileset = sourceFiles; };
-in buildDunePackage rec {
+buildDunePackage rec {
   pname = "ppx_unicode";
   version = "0.1.0";
-  src = sourceSet;
+  src = gitSource { root = ./.; };
 
   minimalOCamlVersion = "5.1";
   dontStrip = true;