The new approach is to not look at RequirementSources at all. Instead,
we exhaustively enumerate all conformance access paths, beginning
from the root conformance requirements in the signature, then doing
all conformance requirements from those protocols' requirement
signatures, and so on.
We enumerate conformance access paths in breadth first order by
length until we find the one we want. The results are memoized.
This fixes a regression with another change I'm working on. The
test case does not fail with this PR alone, but I'm adding it now
anyway.
