Utilize KCModuleLoader::loadModule for loading KCMs
This will take care of showing user-visible error messages in case the plugin does not exist.
While we get a log message from KCoreAddons with the change of 728b4498, the user does not get prompted. Especially if the kcmshell/systemsettings was not opened from the command line, spotting those error messages is more difficult.
Depends on frameworks/kcmutils!74 (merged), otherwise we could run into the assertion that got removed in KCMUtils.