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

third_party/def_parser/BUILD

@@ -1,3 +1,5 @@
+load("@io_bazel//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("@io_bazel//tools/python:private/defs.bzl", "py_library")
 
 filegroup(
     name = "srcs",

third_party/protobuf/3.6.1/protobuf.bzl

@@ -1,3 +1,5 @@
+load("@io_bazel//tools/python:private/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/six.BUILD

@@ -1,3 +1,5 @@
+load("@io_bazel//tools/python:private/defs.bzl", "py_library")
+
 genrule(
   name = "copy_six",
   srcs = ["six-1.10.0/six.py"],

third_party/py/abseil/BUILD

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

third_party/py/concurrent/BUILD

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

third_party/py/gflags/BUILD

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

third_party/py/mock/BUILD

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

third_party/py/six/BUILD

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