Port KCM querying from KPackageLoader to KPluginMetaData

Respect the new namespaces for modules and query them using KPluginMetaData. This avoids kinfocenter KCMs showing up when not wanted.
By using the embedded json metadata of the plugins we don't need to look into multiple namespaces when instantiationg the KCM.

Simplify code using KPluginFactory::instantiatePlugin while at it, this method takes also care of logging errors.
Also it makes sure the factory has the metadata set, before the plugin is created.

Task: https://phabricator.kde.org/T14517