uv-resolver: fix marker propagation
This PR represents a different approach to marker propagation in an
attempt to unblock #4640. In particular, instead of propagating markers
when forks are created, we wait until resolution is complete to
propagate all markers to all dependencies in each fork. This ends up
being both more robust (we should never miss anything) and simpler to
implement because it doesn't require mutating a `PubGrubPackage` (which
was pretty annoying). I think the main downside here is that this can
sometimes add markers where they aren't needed.
This actually winds up making quite a few snapshot changes. I went
through each of them. Some of them look like legitimate bug fixes. Some
of them look like superfluous additions. And some of them look like they
would be removed if we had perfect marker normalization. But I don't
think any of the changes are _wrong_.
