Don't use KPluginInfo if we can use KPluginMetaData

Summary: KPluginInfo would force the load of the desktop file for information we don't need.

Test Plan: Plasma still works

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22480