Per-package-target feature unification

Hi,

For our embedded development workspaces, I was looking forward to using per-package-target... But this issue remaining with that: per-package-target unifies features which can conflict with other targets · Issue #9521 · rust-lang/cargo · GitHub is quite the problem for us (since we get std flags set for the embedded build...).

I've had a stab at fixing the issue: Issue · GitHub and was wondering what the proper way would be to go forward with that. My fix doesn't seem to far out of place, but my knowledge on the codebase is quite limited - I was unsure about opening a new issue with the old one being present, but necroing that old issue doesn't seem like it was such a great idea either...

It would be cool to get some feedback, I'm happy to spend the time to work on this to make it fit the places/cases I don't understand yet.

In general there doesn't yet seem to be an accepted way forward - would fixes then still be accepted?