There doesn't appear to be any exposed object that will show you the relationship between the Content Pack ContentVersion and the member ContentVersion records.
As you have found, you can find the ContentVersion record based on the FileType being 'PACK'. You can also see the ContentVersion records that make up the pack. But there is currently no way to see the the connecting relationship.
"You can't create, edit, or delete content packs via the API."
Source: Online Docs - ContentDocument
The following Idea includes exposing the relationship in the API:
Show Where Content is Used in Content Packs or Deliveries.
Users cannot delete Content that is part of some other Content Pack. They are notified but it seems they have no way to see which Content Packs are in question. Let's put this information on the Content's Detail Page, perhaps in a related list. That way, users could notify Content Pack owners to remove or update the items from the packs. Let's make this function from the API and Reports too.