Consider Transformer::explicitlyDependsOn when disconnecting artifacts
Consider Transformer::explicitlyDependsOn when disconnecting artifacts

This was forgotten in d0e8197bbf. As a result, transformers could hold
references to invalid artifacts, resulting in a crash when storing the
build graph.

Change-Id: I6af0d15d0c0ad44f1f9011c88e413f327a05943d
Reviewed-by: Joerg Bornemann <47777b9406dbddd02a3fdca8341789a013805a11@qt.io>

qt-labs/qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs