Create less draconian loops for grouped collections

Create less draconian loops for grouped collections
- Instead of creating a completely unresolvable tangle of pre-requisites
  between all collection members and their requirements, just add
  a single relation loop of all the packages in a (grouped) collection,
  effectively turning it into a strongly connected component. The
  loop-cutting algorithm has slightly better chances of surviving this
  while still enforcing the collection members to be installed in one
  lump. Also only process each collection just once.