manager: nits, cleanups, fixes

See commit messages.

In particular, this fixes poe check on my system, where the build.py collides with the Python build library used by (some versions of?) Poetry. Not sure if that is a new thing or what is happening here, but there have been problems reported before.

I still think I was right that this package should have never been named "build" 😛
- layday, https://github.com/pypa/build/issues/526, 2022