I went down this route because I figured the installer binary should be in `check_deps`. The result:
- Add thin wrapper scripts for `buildozer` and `bazel`, similar to the `buildifier` wrapper that's already there.
- These are mainly to avoid deduplicating logic, and dodging intricacies of Python imports that I don't understand well through instead subprocessing.
- Fix update_roots.py (it was missing a `.parent`, must've been relocated without a re-run at some point)
- Fix fuzzer proto code to be `testonly`
- Update the `check_deps` targets
Moves common script logic into utils.py (not a great name, but couldn't come up with better). This is in particular to make the buildifier.py script really trivial, allowing that pre-commit to be easily added. However, scripts have also been diverging on how we find bazel, so I'm trying to unify that.
The advantage of reimplementing buildifier's pre-commit is that (a) we can now run buildifier server-side, and (b) we can stop advising installing it manually. Then the only Linux-specific package manager is Cargo, which is only used for watchman, which is optional -- so stop highlighting Linux-specific package managers in the tool instructions.