Auto merge of #6764 - matthiaskrgr:lintcheck_par_iter, r=flip1995

lintcheck: parallelize

By default we use a single thread and one target dir as before.

If `-j n` is passed, use `n` target dirs and run one clippy in each of them.
We need several target dirs because cargo would lock them for a single process otherwise which would prevent the parallelism.
`-j 0` makes rayon use  $thread_count/2 (which I assume is the number of physical cores of a machine) for the number of threads.

Other change:
Show output of clippy being compiled when building it for lintcheck (makes it easier to spot compiler errors etc)
Show some progress indication in the "Linting... foo 1.2.3"  message.
Sort crates before linting (previously crates would be split randomly between target dirs, with the sorting, we try to make sure that even crates land in target dir 0 and odd ones in target dir 1 etc..)

*Please write a short comment explaining your change (or "none" for internal only changes)*
changelog: parallelize lintcheck with rayon

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