Visit source distributions before wheels (#10291)
## Summary
This should address the comment here:
https://github.com/astral-sh/uv/pull/10179#issuecomment-2569189265. We
don't compute implied markers if the marker is already `TRUE`, and we
set it to `TRUE` as soon as we see a source distribution. So if we visit
the source distribution before the wheels, we'll avoid computing these
for any irrelevant distributions.
