The reason behind inflated compilation times in `deep-and-broad`

This is a guess:

Old incremental builds seem not to be cleanup when the dependencies change. That leads to a quadratic number of remnant folders of incremental states. Since the incremental engine iterates *all of* them when finding a suitable build folder to pick off (and finds none), this will do a quadratic number of io actions.

Reasoning:

After executing the creation script for a while we find a ton of remanining incremental directories

```bash
$ find -type d -name 'deeper*' target/incremental | wc -l
5402
```

The issue describes a potential cause of inflated compilation times in the `deep-and-broad` scenario, likely related to incremental compilation in rustc/cargo. Start by examining the script that creates the scenario and the target/incremental directories it leaves behind. Investigate how cargo manages incremental build folders and when they are cleaned up. Look for the code in cargo or rustc that iterates over these directories. Determine if old incremental states are being retained when dependencies change, leading to quadratic IO.

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