Rollup merge of #136091 - jieyouxu:core-tracing, r=clubby789

Add some tracing to core bootstrap logic

Follow-up to #135391.

### Summary

Add some initial tracing logging to bootstrap, focused on the core logic (in this PR).

Also:

- Adjusted tracing-tree style to not use indent lines (I found that more distracting than helpful).
- Avoid glob-importing `tracing` items.
- Improve the rustc-dev-guide docs on bootstrap tracing.

### Example output

```bash
$ BOOTSTRAP_TRACING=bootstrap=TRACE ./x check src/bootstrap
```

![Example bootstrap tracing output](https://github.com/user-attachments/assets/0be39042-0822-44b6-9451-30427cfea156)

r? bootstrap

0 files changed, 0 insertions, 0 deletions