support Qt File selectors in KPackage

Summary:
support for things like contents/ui/+phone/main.qml
right now in qml based packages qml file selectors are supported as
there is always one internal for qml, for every file, *except*
main.qml or any loaded directly by kpackage.
this gives the same behavior between main.qml and every other file
in the package

Test Plan:
package without selector folders works exactly the same and every plasmoid
kcm and so on is loaded fine.
if one adds for instance in a kcm package under ui the subfolder +phone with a
main.qml, then export QT_FILE_SELECTORS=phone, that alternative qml file will be
loaded instead

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25394