Fix wrong "Unable to find service type" warnings

Summary:
If we parsed the service type from a qrc file (like kpackagetool does)
then it's wrong to later on say we can't find the file defining that
service type.

Test Plan:
make uninstall in kpackage, then
XDG_DATA_DIRS=$KDEDIR/share <builddir>/bin/kpackagetool5 --appstream-metainfo . --appstream-metainfo-output /tmp/a.xml

Used to say: Unable to find service type for service "KPackage/Generic" listed in "./metadata.desktop"
even though it parsed ":/kservicetypes5/kpackage-generic.desktop" before.

For this we need to extract and store the name of the service type we're
parsing (key "X-KDE-ServiceType").

Reviewers: mart, arichardson, davidedmundson, mpyne, apol

Reviewed By: apol

Subscribers: anthonyfieroni, aacid, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D20748