Ignore direct URL distributions in prefetcher (#2943)
## Summary
The prefetcher tallies the number of times we tried a given package, and
then once we hit a threshold, grabs the version map, assuming it's
already been fetched. For direct URL distributions, though, we don't
have a version map! And there's no need to prefetch.
Closes https://github.com/astral-sh/uv/issues/2941.
