uv-resolver: fix conflict marker simplification bug
The underlying cause here, I believe, was that we weren't accounting
for the case where an edge could be visited *without* any extras
enabled. Because of that, we got into situations where we thought
there was only one path to an edge when there were actually more
paths. This in turn lead to us erroneously doing simplification where
it actually isn't justified. And in turn lead to duplicate versions
of the same package being installed in the same environment.
The fix for this ends up being really simple: in the case where we
don't add any conflict items for a package during graph traversal,
we materialize an empty set of conflicts to mark the case of no
extras being enabled when visiting the child edges. This is enough
to propagate the knowledge of multiple paths to the same edge and
causes us to avoid doing improper simplifications.
This does fix the problem in the snapshot, but it does also I think
lead to other cases where simplifications are no longer possible
(hence the changes to the airflow snapshot). But this seems
expected, since we are doing strictly less simplification than we
were before. It's unclear if all of those cases were actual bugs
or not though.
