Luci builder configs have now been available in separate repos. This CL enables validation of the config json file format for packages recipe.

Change-Id: Ie4ffbac6a80a473b67525ba49a96513669d73d9f
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/5281
Reviewed-by: Tong Wu <wutong@google.com>
Commit-Queue: Keyong Han <keyonghan@google.com>