use KPackage::fileUrl to support rcc KCMs packages

Summary:
the previous code would construct file:// URIs. instead, leave the url
construction to kpackage so we get either file:// or qrc:// depending on
the actual type of the package

Test Plan:

• cmake pass
• make pass
• make test pass

Reviewers: mart, apol

Reviewed By: apol

Subscribers: apol, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D19385