Use ecm_setup_qtplugin_macro_names

Summary:
Avoids all the explicit boilerplate needed, ensures the dependency on
the JSON file ia also set and allows simple export of the setup
in the installed CMake config file, so other projects importing
the config file have the respective CMake macros set as recommended.

Reviewers: dvratil, mlaurent

Reviewed By: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D28640