loading: clean up more concurrency issues (#56329)
Guarantee that `__init__` runs before `using` returns. Could be slightly
breaking for people that do crazy things inside `__init__`, but just
don't do that. Since extensions then probably load after `__init__` (or
at least, run their `__init__` after), this is a partial step towards
changing things so that extensions are guaranteed to load if using all
of their triggers before the corresponding `using` returns
Fixes #55556
