move artifacts set at module level to avoid recomputation

Author: darinyu

metaflow/flowspec.py

@@ -28,6 +28,8 @@
 
 from .datastore.inputs import Inputs
 
+INTERNAL_ARTIFACTS_SET = set(["_foreach_values"])
+
 
 class InvalidNextException(MetaflowException):
     headline = "Invalid self.next() transition detected"
@@ -419,7 +421,6 @@ def merge_artifacts(
             This exception is thrown in case an artifact specified in `include` cannot
             be found.
         """
-        INTERNAL_ARTIFACTS_SET = set(["_foreach_values"])
 
         include = include or []
         exclude = exclude or []