Suggest alternative toolchains for unavailable targets

[Sreehari425]

Closes #4164

rustup target remove <target> now suggests another installed toolchain when the target isn't available in the current one. If no other toolchain has the target, the existing error is unchanged.

[djc]

Uh, this seems to have a bunch of unrelated changes, and lost the useful commit history split in the process.

[Sreehari425]

Uh, this seems to have a bunch of unrelated changes, and lost the useful commit history split in the process.

yeah you are right sorry. I was trying to make the target changes into one clean commit and accidentally pulled the unrelated changes into it too. I messed up the history, fixing it now rn

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[Sreehari425]

@djc thanks for reviewing this earlier :), i have addressed the issues you pointed out and cleaned up the changes would appreciate another look when you get a chance.