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.