SolutionDescriptor should know the set of factClass, so that CS.from() can use those for check

The InnerConstraintFactory should not do any reflection, including the call to extractCollectionGenericTypeParameter().

Proposal A)
SolutionDescriptor has a factClassSet (a fact is either a problem fact or a planning entities).

Proposal B)
SolutionDescriptor has a problemFactClassSet (and it already entityDescriptorMap.keySet() for the entities)