Install KPackages with kpackage_install_package

Summary:
Removes a bunch of boilerplate CMake code and allows kpackage to do
some smart things, e.g. drops our runtime dependency on the
DesktopFileParser and we get to just use json directly.

Test Plan: Ran kwin, now it doesn't use the desktop to json translation path, everything still works.

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: davidedmundson, zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D22474