qt.conf still expects entries called "Qml2Imports" rather than "QmlImports"

The "2" is a vestige from Qt5 having to discern between two versions of the QML language. It should go away.

In an ideal world we could just rename it, but we have to somehow support qt.conf files with the old name. At least there should be a way to make QmlImports an alias of Qml2Imports. However, as qt.conf is probably read from many places, and the keys are exposed in our API, that still requires a migration path.