Dev/pack ref

Actually, I used `system` for system content packs so this should be changed to `system` or the code updated to use `core` for the system level packs.

If we decided to go with the latter, it will require a bit more changes and testing. Currently in some places, we validate that the name of the user content pack is not `system`. I just need to check the code and make sure that we don't do the same for core packs in `contrib/core/`, otherwise the registration of those packs will fail.