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

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