Review 3rd party plugin-engine implementations in replacement of `byexample`'s one

**Describe the feature you'd like**
So far `byexample` uses a home-made plugin engine. In `byexample` 10.5.0 we had to change it to use the new import-mechanism of Python 3.10 and it wasn't the smoothest experience.

In #220 we found that modules loaded in this way are **not** pickle-able by default and they are not compatible with the way that `multiprocessing` *spawn* sub processes.

Moreover the current plugin-engine has a poor support plugin versioning, conditional loading, and loading order (for dependencies).

This ticket if for review which alternatives exist.