Plugin info objects are owned by the plugin manager (composition
pattern). Even if the plugin manager is a singleton, we need to keep
explicit reference to the plugin manager that owns the plugin info so
that we can mock up things in unit tests.

Therefore this patch introduces a new property of plugin info objects
that holds the reference to the instance of the plugin manager that made
(and hence owns) the given info instance.

The only trouble here is with static methods of plugin info classes such
as \core\plugininfo\base::get_enabled_plugins(). In these cases, the
code keeps using the core_plugin_manager singleton. The solution would
be to pass the plugin manager instance as a parameter but that is not
worth of change for now, IMHO.