Do not report an error for a non-existing file in a disabled group.

Task-number: QBS-470

Change-Id: I839c5778bbc37ffb907315487bd1a072ba9f1791
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Remove "Module" from the list of allowed child types for some items.

While we do create Item objects for module dependencies, these are not
actually added as children.

Task-number: QBS-507
Change-Id: I923cea5b3d770b620fb3682be717d12b73058045
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>