Fix artifact cleaner.

If the "remove all" flag is not set, we must take care to not only skip
target artifacts, but also their siblings that are not target artifacts.
Otherwise, the build graph can end up in an unexpected state.

Change-Id: Ie2fa36f3e489c3bee19f461e16fa093515b01020
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>