setup: run require_auto_deps() before attempting to import any deps in __init__.py

setup: run require_auto_deps() before attempting to import any deps in __init__.py
For one thing, this makes missing-dependency failures into DistributionNotFound errors instead of ImportErrors, which might be more useful to the user. For another thing, if someone is using distributions that were installed with --multi-version, then they might be not importable until after require_auto_deps() has been run. (The docs claim that this would be the case, but we don't have an example of this happening at this time.)