WorldSelectedSet._load: load all nested atoms/sets

Since commit 3e327e8c849cf6bfb84a3a3ec5c080ab4bc4653c,
AutounmaskTestCase.testAutounmaskAndSets fails due to incorrect logic
in WorldSelectedSet._load. Fix it to load all nested atoms/sets.

Fixes: 3e327e8c849c ("Split @selected into @selected-packages and @selected-sets")