Refactor entities/common

Extract shadowing check from make_items_product_specific. It supposed
to lower the cognitive complexity of the function.

ssg/entities/common.py

@@ -19,6 +19,10 @@
 )
 
 
+def item_is_shadowed(items_dict, label, value):
+    return label in items_dict and value != items_dict[label]
+
+
 def make_items_product_specific(items_dict, product_suffix, allow_overwrites=False):
     new_items = dict()
     for full_label, value in items_dict.items():
@@ -43,7 +47,7 @@ def make_items_product_specific(items_dict, product_suffix, allow_overwrites=Fal
         if not full_label.endswith(product_suffix):
             continue
 
-        if label in items_dict and not allow_overwrites and value != items_dict[label]:
+        if not allow_overwrites and item_is_shadowed(items_dict, label, value):
             msg = (
                 "There is a product-qualified '{item_q}' item, "
                 "but also an unqualified '{item_u}' item "