reuse KPackage instance between PluginLoader and Applet

Summary:
to minimize metadata lookups, pass the kpackage created in pluginloader
to the applet itself as args. the args structure will be used only by the
internal constructor and applet sublasses will get their args unaltered

Test Plan:
plasma loads correctly, from 3 kpackage instantiation per applet to 2
(last is in the access manager)

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D9123