peer optional failure D

root -> (x, z@1)
x -> PEEROPTIONAL(y)
y -> PEER(z@2)

npm/arborist#223

npm/arborist#236

Subtly similar to case A, but note y and z swapped, so that they are resolved in the reverse order (because deps are alphabetically sorted for consistency). This is relevant, because it ensures that there's no dep present in the peerSet until after the conflict is encountered, and that code path was not being hit.