Fixes #25813.
I initially tried doing this in `deno_semver`, where it's a cleaner
change, but that caused breakage in deno in places where we don't expect
a tag (see https://github.com/denoland/deno/issues/25857).
This does not fix wildcard requirements failing to choose pre-release
versions. That's a little more involved and I'll do a separate PR.
