Previously it required:
  noIntersection processingClosure failedSet

Lets see what it's saying and why it's wrong. Suppose we have P1 and P2
in the processing set. We have Q that depends on P1 and P2. So the
processingClosure is {P1,P2,Q}. Now suppose building P1 fails. The
failedSet is now {P1,Q1}, and the processingSet is {P2}. The
processingClosure however is {P2,Q}. Thus we have a non-empty
intersection between the processingClosure and failedSet, namely {Q}.

So clearly it's bogus, but also it's not clear if I was thinking of
something else that is correct. So we just drop this part of the
invariant.

This should fix issue #3889.