We sometimes have two alternatives a.m and b.m with the same symbol
but different prefixes. Previously these would always be ambiguous.
We now try to disambiguate this so that the alternative with the
more specific prefix wins. To determine this, we widen prefixes
also going from module classes to their parents and then compare
the resulting types.

This might fix a problem in ScalaTest that popped up after scala#20054.

compareOwner did certain tests for one side but not for the other, which made its
outcome dependent on the order in which alternatives were presented.

The alternatives with the same symbol could have nevertheless different types. We first want
to disambiguate based on these types before we turn to prefixes as a final tie breaker.