[Serialization] Fix missing lookup of package-only dependencies in swiftmodule

I don't think this logic is used in practice without merge-modules.
Let's still implement it for the configurations still using
merge-modules and for general consistency.

lib/Serialization/ModuleFile.cpp

@@ -468,6 +468,10 @@ void ModuleFile::getImportedModules(SmallVectorImpl<ImportedModule> &results,
         continue;
       }
 
+    } else if (dep.isPackageOnly()) {
+      if (!filter.contains(ModuleDecl::ImportFilterKind::PackageOnly))
+        continue;
+
     } else {
       if (!filter.contains(ModuleDecl::ImportFilterKind::Default))
         continue;