Use ecm_setup_qtplugin_macro_names

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.

Test Plan:
Still builds as before. Touch a plugin metadata JSON file now triggers a moc
rerun. CMake config file contains setup of variables.