Per-package-target feature unification

> In general there doesn't yet seem to be an accepted way forward - would fixes then still be accepted? The Cargo team notably cares about getting the design sorted out before accepting patches, so you should talk with them, on #t-cargo on Zulip and perhaps the Cargo Office Hours video call. Or perhaps right here will do. But in any case, you should probably write a document explaining the design of your fix.