Auto merge of #9268 - ehuss:2021-fix-edition-v2-warning, r=alexcrichton

Auto merge of #9268 - ehuss:2021-fix-edition-v2-warning, r=alexcrichton

Add report if `cargo fix --edition` changes features.

This adds a small report if using `cargo fix --edition` to transition from 2018 to 2021 to determine if the new resolver will result in different features.

There is a gauntlet of checks that must pass before it even tries to show the differences:
* If the resolver is already specified, skip.
* If in a virtual workspace, skip (no way to specify global edition).
* If not migrating the root package, skip. The edition only changes the resolver in the root package.
* If not migrating all targets, skip. It's uncertain if the user intends to set the package edition or not.

Closes #9048