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 instantiatiating the KCM.
Simplify code using KPluginFactory::instantiatePlugin while at it, this method takes also care of logging errors.
Task: https://phabricator.kde.org/T14517
I will work on putting the plasma KCMs in the new namespaces after this lands. The KInfoCenter ones have already been taken care of.