Mark kpackage_install_package undeprecated for plasma_install_package

Summary:
While no-one is working on porting plasma_install_package away from
kpackage_install_package, and given this results in long logs of
deprecation warnings for Plasma repo, for now enable
plasma_install_package to disable the deprecation warning with a
undocumented flag.

Reviewers: mart, apol

Reviewed By: mart

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D11129