Disable deprecation warning from kpackage_install_package for now

Summary:
Using a private flag just added for plasma_install_package, until
there is a new official replacement for kpackage_install_package for
its needs

Test Plan: kdeplasma-addons & Co still build and now have clean cmake log again.

Reviewers: mart, apol

Reviewed By: mart

Subscribers: Frameworks

Tags: Frameworks, Plasma

Differential Revision: https://phabricator.kde.org/D11130