Free the registered extension impls when the object module is unloaded

When the object module is reloaded they will all be added again as
peas_register_types() will be called.

This caused problems when checking if the interface was already
registered as it was always already registered when reloading the
module.
Warn and fail when registering an extension impl multiple times

Registering an extension impl multiple times will not have the
desired effect as only the first one registered will ever be used.