NPE in ResolverImpl

The method getPackageSources in ResolverImpl.java returns a Set.
This collection is used in mergeUses (line 1039) in a foreach loop.

A null value may be returned by getPackageSources, as a result of querying a Map. In this case, the foreach loop in mergeUses will trigger a NPE.

The method getPackageSources should return an empty Set when there is no result.