[kcmkwin/kwindesktop] Use PackageLoader to get effect metadata

Summary:
`KPluginInfo(KService::Ptr)` is deprecated in favor of using plugin
loaders.

Test Plan:
* Go to System Settins -> Desktop Behavior > Virtual Desktops
* Open Switching tab
* Select Fade desktop
* Click the info button

Reviewers: #kwin, apol, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D13851