chore: don't add migration tag if Starlark implementation is enabled

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

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):