use the new Package::fileUrl to load from resources

Summary: QUrl::fromLocalFile can't be used when packages are qrc

Test Plan: plasashell loads correctly with packages from qrc

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9176