Prevent sibling artifacts from getting different transformers.

It is a strict requirement that all output artifacts in a rule
point to the same Transformer object. This invariant can currently
break under very specific circumstances related to build graph
change tracking, which is likely the cause of QBS-379.

Task-number: QBS-379
Change-Id: I85dc28e69794df5f50c5155171b393ab14ca6dbc
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>