Avoid duplicates on WorkOrderPackageReceiveSelectionStep

Use .add_list instead of .extend, so it doesn't get duplicates when
the user does to the next step and them come back.

Change-Id: Ia99b70d99afe2892c9d68e61c629f1c522d0d7d7
Merge "Avoid duplicates on WorkOrderPackageReceiveSelectionStep"