Auto merge of #18181 - davidbarsky:davidbarsky/push-nzstpumovmmx, r=davidbarsky

internal: add tracing to project discovery and VFS loading

With `"env RA_PROFILE=vfs_load|parallel_prime_caches|discover_command>500`, this results in the following output:

```
21888ms discover_command
11627ms vfs_load @ total = 701
1503ms vfs_load @ total = 701
30211ms parallel_prime_caches
```

As a followup, I'd like to make hprof emit the information above as JSON.

Author: bors