audit registrations in extension packages for parallelism compatibility

[simonpcouch]

If a model specification in an extension package uses set_dependency() on its engine package but not on the extension package itself, the model specification will not be available in socket cluster (i.e. plan(multisession)) parallel workers, e.g. https://github.com/tidymodels/bonsai/pull/86. We can likely put together a quick script to check for other engines where this is the case.