Optional resource fragment with requirements that cause class space consistency issues with the host export cause unexpected ResolutionExceptions

Not sure if this is actually a bug in version 1.6.0. I am testing with the latest out of trunk so it could be something after 1.6.0.

If a fragment contains a requirement that ultimately causes a uses constraint conflict with one of the hosts exported packages then the resolver will end up throwing a ResolutionExceptoin even when the fragment and host are considered to be optional resources by the ResolverContext.