Adding support for KPackage directly to KNewStuff means that we are
able to deal more gracefully with things like Plasma's Global Themes
(and indeed any other kpackage based thing).
This is done by adding another archive specialisation to the installer
class, and by also adding a check to the cache to ensure that even
when a kpackage is removed from the system outside of KNewStuff,
it does not remain seemingly installed in the KNS lists.
* Make sure the cache gets written
* Add KPackage support to KNSCore::Installation
BUG:418466