Auto merge of #44792 - Mark-Simulacrum:correct-deps, r=alexcrichton
Make sure to clear out the stageN-{rustc,std,tools} directories.
We copy built tool binaries into a dedicated directory to avoid deleting them,
stageN-tools-bin. These aren't ever cleared out by code, since there should be
no reason to do so, and we'll simply overwrite them as necessary.
When clearing out the stageN-{std,rustc,tools} directories, make sure to delete
both Cargo directories -- per-target and build scripts. This ensures that
changing libstd doesn't cause problems due to build scripts not being rebuilt,
even though they should be.