Auto merge of #145295 - Kobzol:unify-stages, r=jieyouxu

Consolidate stage directories and group logs in bootstrap

My post-stage-0-redesign bootstrap fixes aren't done yet, but I think that enough steps have been migrated to the new system that it makes sense to actually modify the directories on disk, and what gets printed when bootstrap runs, so that it actually corresponds to the new system. Before, the printed stages didn't always make sense.

This PR:
- Fixes the numbering of `stageN` directories in the build directory. It was not corresponding to the correct stages before; notice that I did not modify `bootstrap/README.md`, as it was essentially describing what happens after this PR (first commit).
- Unifies all steps that output a build group to use the `Builder::msg` method. It's probably not the final stage, and some of the test steps might not be fully accurate yet, because I didn't fix test step numbering yet, but I think that it's a clear improvement from before, and now that everything uses the same method, we can easily make changes across the board, to ensure that it stays unified (second commit).

r? `@jieyouxu`

try-job: dist-x86_64-msvc
try-job: dist-x86_64-linux

Author: bors