refactor ExcludeNewer to avoid accidentally exposing "local date" input #6820
Description
Basically, see: #6562 (review)
Right now, we are relying on ExcludeNewer in uv-resolver to have different implementations for Deserialize and FromStr in order to avoid allowing "local" dates in the [tool.uv] section. That seems very easy to accidental mess up, so we should either at least add a comment or change the code to make this harder to mess up.