Reword debug log messages for QML import statements

Setting the environment variable QML_IMPORT_TRACE=1, we get messages like

qt.qml.import: addLibraryImport: qrc:/MyApp/main.qml "QtQuick" version ' invalid ' as ""

The word "invalid" can give the impression that a problem has occurred. Since it is perfectly valid (and recommended) to write "import QtQuick" instead of "import QtQuick 2.15", I suggest changing the log message (perhaps "unversioned" instead of "invalid")