Remove ignore for F401 in __init__.py files and fix violations

See individual commits.

added type::fix label

Why is

from pmb.build.init import init as init

better than

from pmb.build.init import init?

It seems to be more verbose to achieve the same thing?

LGTM! Waiting a bit in case @whynothugo or @caleb have further comments, otherwise let's get this merged!

approved this merge request

resolved all threads

added 20 commits

• ade951da...63fce4fc - 16 commits from branch master
• 06202328 - pmb: Remove unused imports (MR 2545)
• bdcc188f - pmb: Use explicit re-exports (MR 2545)
• ebc5ab9e - pmb.chroot: Rename exists() to user_exists() (MR 2545)
• 5f8d0141 - pyproject.toml: Remove ignore for F401 in __init__.py files (MR 2545)

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enabled an automatic merge when all merge checks for 5f8d0141 pass

merged