Fix bug in rescuing build data.

We tried to get information about an artifact's children from the
artifact itself. However, that artifact could have gotten disconnected
along the way and not know anything about its children anymore.
We must therefore save such information in an extra data structure.

Change-Id: Id2043f97e857bf8779184e88b1425fdcf85e3f2e
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>