Add load() statements for Python rules in third_party

This replaces all direct uses of the native Python rules underneath the
third_party/ directory with load()s of the internal wrapper macros. These
macros are needed for compatibility with
`--incompatible_load_python_rules_from_bzl`.

Work toward bazelbuild#9006.

RELNOTES: None

Author: brandjon

WORKSPACE

@@ -79,6 +79,15 @@ new_local_repository(
     path = "./third_party/protobuf/3.6.1/",
 )
 
+# Used by protobuf.
+# TODO(#9029): This should be replaced by the real bazelbuild/rules_python.
+# Also, all non-@bazel_tools uses of Python rules in the Bazel source tree
+# should be updated to reference @rules_python//python:defs.bzl.
+local_repository(
+    name = "rules_python",
+    path = "./third_party/rules_python",
+)
+
 local_repository(
     name = "googleapis",
     path = "./third_party/googleapis/",

third_party/def_parser/BUILD

@@ -1,3 +1,5 @@
+load("//tools/python:private/defs.bzl", "py_test")
+
 licenses(["notice"])  # 3-clause BSD
 
 package(

third_party/protobuf/3.6.1/BUILD

@@ -21,6 +21,7 @@ config_setting(
 
 load(":protobuf.bzl", "py_proto_library")
 load(":compiler_config_setting.bzl", "create_compiler_config_setting")
+load("@rules_python//python:defs.bzl", "py_library")
 
 filegroup(
     name = "srcs",

third_party/protobuf/3.6.1/protobuf.bzl

@@ -1,3 +1,5 @@
+load("@rules_python//python:defs.bzl", "py_library", "py_test")
+
 def _GetPath(ctx, path):
     if ctx.label.workspace_root:
         return ctx.label.workspace_root + "/" + path
@@ -383,7 +385,7 @@ def py_proto_library(
     if default_runtime and not default_runtime in py_libs + deps:
         py_libs = py_libs + [default_runtime]
 
-    native.py_library(
+    py_library(
         name = name,
         srcs = outs + py_extra_srcs,
         deps = py_libs + deps,
@@ -406,7 +408,7 @@ def internal_protobuf_py_tests(
     """
     for m in modules:
         s = "python/google/protobuf/internal/%s.py" % m
-        native.py_test(
+        py_test(
             name = "py_%s" % m,
             srcs = [s],
             main = s,

third_party/protobuf/3.6.1/python_defs.bzl

@@ -0,0 +1,58 @@
+# Copyright 2019 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""This is a Bazel-internal file; do not load() it!
+
+The incompatible change `--incompatible_load_python_rules_from_bzl` (#9006)
+makes it so the four native Python rules cannot be used unless a magic tag is
+present. It is intended that only `@rules_python` (bazelbuild/rules_python)
+uses this tag, and all other uses access the native rules via the wrapper
+macros defined in `@rules_python`.
+
+However, `@bazel_tools` is not allowed to depend on any other repos. Therefore,
+we replicate the behavior of `@rules_python`'s wrapper macros in this file, for
+use by Bazel only.
+
+This gets a bit tricky with the third_party/ directory. Some of its
+subdirectories are full-blown workspaces that cannot directly reference this
+file's label. For these cases we currently copy this file into those trees
+under the name `python_defs.bzl`.
+
+TODO(#9029): Currently all uses of Python in Bazel's source tree use this file,
+but in principle only `@bazel_tools` needs to. The rest should use the real
+`@rules_python`, or for third_party/ packages that get mirrored into
+`@bazel_tools`, a dummy version of the repo that contains only this file under
+label `@rules_python//python:defs.bzl`.
+"""
+
+_MIGRATION_TAG = "__PYTHON_RULES_MIGRATION_DO_NOT_USE_WILL_BREAK__"
+
+def _add_tags(attrs):
+    if "tags" in attrs and attrs["tags"] != None:
+        attrs["tags"] += [_MIGRATION_TAG]
+    else:
+        attrs["tags"] = [_MIGRATION_TAG]
+    return attrs
+
+def py_library(**attrs):
+    native.py_library(**_add_tags(attrs))
+
+def py_binary(**attrs):
+    native.py_binary(**_add_tags(attrs))
+
+def py_test(**attrs):
+    native.py_test(**_add_tags(attrs))
+
+def py_runtime(**attrs):
+    native.py_runtime(**_add_tags(attrs))

third_party/py/abseil/BUILD

@@ -1,3 +1,5 @@
+load("//tools/python:private/defs.bzl", "py_library")
+
 licenses(["notice"])
 
 filegroup(

third_party/py/concurrent/BUILD

@@ -1,3 +1,5 @@
+load("//tools/python:private/defs.bzl", "py_library")
+
 licenses(["notice"])
 
 filegroup(

third_party/py/gflags/BUILD

@@ -1,3 +1,5 @@
+load("//tools/python:private/defs.bzl", "py_library")
+
 licenses(["notice"])
 
 filegroup(

third_party/py/mock/BUILD

@@ -1,3 +1,5 @@
+load("//tools/python:private/defs.bzl", "py_library")
+
 licenses(["notice"])
 
 filegroup(

third_party/py/six/BUILD

@@ -1,3 +1,5 @@
+load("//tools/python:private/defs.bzl", "py_library")
+
 licenses(["notice"])
 
 filegroup(