There's no real point in maintaining a separate `MMPlugin' interface, as all the
plugins will inherit from `MMPluginBase', so just merge them and simplify
everything.
Before this, we only exported the modem to DBus when all ports were organized,
in order to make sure that we select as primary port the one we really want and
not the first AT port grabbed. Given that to get all the ports organized we also
needed to wait to get all the ports grabbed, we can now also defer the creation
of the modem object until all the ports get grabbed. This allows us to create
different types of objects based on the ports available (e.g. we can now create
QMI-supported modem objects if we see a QMI port around).
