Fix artifact visitor.

The current implementation is overly complex and misses sibling
artifacts. Just go through the list of artifacts instead of traversing
top-down.

Change-Id: Ic665bfc977a1718ee298efda66d741bb701886c3
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Fix "clean" operation.

We were missing some artifacts, and we also left some empty directories
behind.

Change-Id: Ib93fb7a6a644e09526c8d4e177d580fe2d870c54
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>