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.