use --debug for the current verbose files output

[juhp]

I find the default output listing every file to delete, too noisy. So I wonder if it would be acceptable to make the default output quieter? If not I can flip this PR to make it an opt-in --quiet option instead...

With this PR the default output

• by default no longer lists each file being deleted for toolchain but just list the project path instead and the total size removed.
• also doesn't warn about unidentified installed toolchain (I don't use rustup but just the rustc from my distro).

'--verbose' might be a better name though it is already taken by cargo

[juhp]

I am afraid I pushed a couple more improvements here: I think they are good too though somewhat separate enhancements.

[jyn514]

@juhp I see quite a lot of unrelated changes in this PR. Could you please split into each of them into a separate PR so I can review / merge them individually? It looks like this PR also contains several breaking changes which I'm not inclined to merge without a strong reason.

[juhp]

Sure, I should have a used branch, before pushing more of my changes...

[jyn514]

I'm going to close this until you get a chance to update it with only the --debug change (which I do think is good and useful). Feel free to reopen once you have time.