Handle universal vs. fork markers with `ResolverMarkers` (#5099)
* Use a dedicated `ResolverMarkers` check in the fork state. This is
better than the `MarkerTree::And(Vec::new())` check.
* Report the timing correct naming universal resolution instead of two
spaces around an empty string when there are no markers.
* On resolution error, show the split that we're in. I'm not sure how to
word this, since we're doing a universal resolution until we fork, so
the trace may contain information from requirements that are not part of
this fork.
