fixup! Use sets

buildstream/_loader/loader.py

@@ -301,10 +301,10 @@ class Loader():
             sequence = []
 
         # Skip already validated branches
-        if validated.get(element) is not None:
+        if element in validated:
             return
 
-        if check_elements.get(element) is not None:
+        if element in check_elements:
             # Create `chain`, the loop of element dependencies from this
             # element back to itself, by trimming everything before this
             # element from the sequence under consideration.