When the dictionary has served its purpose (making plugin loading
pre-and-post init a consistent behaviour), we swap it for bools
indicating whether or not a module was loaded.
We don't currently use the bools, but it seemed a sensible choice.
The main idea is to make the dictionary fully pickle-able, so that when
dill pickles the linter for multiprocessing, it doesn't crash horribly.

Co-authored-by: Daniël van Noord <[email protected]>

This should cover the case where plugins are loaded and parallel running
is enabled.

The test also includes an assert to confirm the module picked is
actually unsafe, in case it is made safe in the future.
Also include docstring edits suggested by reviews.

Running in isolation and as part of the full suite cause different
default plugins to be loaded, which throw different pickling errors.
This will now catch all of them, and still fail the test if all of those
modules become pickle safe in future.

Thanks for the review!