Adapt FindKF5 to stricter checks in newer find_package_handle_standard_args
Test Plan: KF modules configure build as before, same some apps.
Reviewers: Frameworks, Build System, cgiboudeaux
Reviewed By: cgiboudeaux
Subscribers: apol, kde-frameworks-devel, kde-buildsystem
Tags: Frameworks, Build System
Differential Revision: https://phabricator.kde.org/D29097