[ILibrary] make referenced_patterns more robust to cyclical dependencies

masque/library.py

@@ -186,9 +186,9 @@ class ILibraryView(Mapping[str, 'Pattern'], metaclass=ABCMeta):
         # Perform recursive lookups, but only once for each name
         for target in targets - skip:
             assert target is not None
+            skip.add(target)
             if target in self:
                 targets |= self.referenced_patterns(target, skip=skip)
-            skip.add(target)
 
         return targets