When calculating the score in the `SpreadIterator`, the score boost is
proportional to the difference between the current and desired count. But when
there are implicit spread targets, the current count is the sum of the possible
implicit targets, which results in incorrect scoring unless there's only one
implicit target.
This changeset updates the `propertySet` struct to accept a set of explicit
target values so it can detect when a property value falls into the implicit set
and should be combined with other implicit values.
Fixes: #11823
