Add pack check helper function

shared/utils.php

@@ -179,3 +179,7 @@ function uupApiReadJson($path) {
 function uupApiWriteJson($path, $data) {
     return file_put_contents($path, json_encode($data)."\n");
 }
+
+function uupApiPacksExist($updateId) {
+    return file_exists('packs/'.$updateId.'.json.gz');
+}