Track missing components

[nrc]

So that if a component is missing and the user tries to add it, Rustup can explain. Ideally it could also suggest the last release with the component.

[nrc]

I think this should be fixed by rust-lang/rust#53715

I wish it were easier to test this

[nrc]

it could also suggest the last release with the component

this part is not done, and I'm not sure how to implement it

[sunjay]

It would also be really helpful if rustup could ask whether or not you want to continue when upgrading would remove a component you previously had installed. If asking for more input would break stuff, we could also error out when an installation would uninstall a component and have an extra --force flag or something to push the update through anyway.

[kinnison]

I think that current behaviour, especially with profiles and nightly backtracking, solves the intent of this issue.