chore: don't add migration tag if Starlark implementation is enabled (#2257)

This avoids the tag being added when it doesn't need to be, which can
look confusing to users without context about what it means.

Work towards #1361

Author: rickeylev

python/private/BUILD.bazel

@@ -386,7 +386,10 @@ bzl_library(
     visibility = [
         "//:__subpackages__",
     ],
-    deps = ["@bazel_skylib//lib:types"],
+    deps = [
+        "@bazel_skylib//lib:types",
+        "@rules_python_internal//:rules_python_config_bzl",
+    ],
 )
 
 bzl_library(

python/private/util.bzl

@@ -15,6 +15,7 @@
 """Functionality shared by multiple pieces of code."""
 
 load("@bazel_skylib//lib:types.bzl", "types")
+load("@rules_python_internal//:rules_python_config.bzl", "config")
 
 def copy_propagating_kwargs(from_kwargs, into_kwargs = None):
     """Copies args that must be compatible between two targets with a dependency relationship.
@@ -60,7 +61,8 @@ def add_migration_tag(attrs):
     Returns:
         The same `attrs` object, but modified.
     """
-    add_tag(attrs, _MIGRATION_TAG)
+    if not config.enable_pystar:
+        add_tag(attrs, _MIGRATION_TAG)
     return attrs
 
 def add_tag(attrs, tag):