* use the activate/deactivate system from WpObject,
  which allows async activation and error reporting
* drop 'module' property, use 'core' from WpObject

Similar to wp_si_factory_find()

There is no good reason to keep them private

This serves as a base class for daemon plugins, which are implemented
inside modules

After discussing things at the AGL May 2019 F2F meeting
and reflecting on the initial design of WirePlumber,
it became clear that it needed a fresh start.

This makes it more intuitive to get other attached interfaces
from the core