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`.

Some of the uses are replaced by the file under the tools/ dir, which is
already used elsewhere in Bazel. A few uses have to use a new @rules_python
repo (see also #9029).

Work toward #9006.

RELNOTES: None

third_party/def_parser/BUILD

@@ -1,3 +1,7 @@
+# Needed for #9006. Consider replacing label with @rules_python as per #9029 if
+# merging from upstream becomes an issue.
+load("//tools/python:private/defs.bzl", "py_test")
+
 licenses(["notice"])  # 3-clause BSD
 
 package(

third_party/protobuf/3.6.1/BUILD

@@ -21,6 +21,8 @@ config_setting(
 
 load(":protobuf.bzl", "py_proto_library")
 load(":compiler_config_setting.bzl", "create_compiler_config_setting")
+# Needed for #9006. Hopefully a future upstream version will include this line.
+load("@rules_python//python:defs.bzl", "py_library")
 
 filegroup(
     name = "srcs",

third_party/protobuf/3.6.1/protobuf.bzl

@@ -1,3 +1,6 @@
+# Needed for #9006. Hopefully a future upstream version will include this line.
+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 +386,7 @@ 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 +409,7 @@ 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 +1,6 @@
+# Needed for #9006. Hopefully a future upstream version will include this line.
+load("@rules_python//python:defs.bzl", "py_library")
+
 genrule(
   name = "copy_six",
   srcs = ["six-1.10.0/six.py"],

third_party/protobuf/README.md

@@ -43,6 +43,11 @@ You will create and merge the following Pull Requests.
     copy the `licenses` declaration and the `srcs` filegroup to the corresponding file under
     `third_party/protobuf/<new_proto>`.
 
+1.  Modify the new protobuf to be compatible with #9006 (until upstream protobuf migrates):
+    In the `BUILD`, `protobuf.bzl`, and `six.BUILD` files under `third_party/protobuf/<new_proto>`,
+    copy the line that loads from `@rules_python` along with its explanatory comment, from the
+    corresponding files in the old proto directory. (See also #9019.)
+
 1.  In `third_party/protobuf/<new_proto>/BUILD`, in the `srcs` filegroup rule, update the version
     number referring to the newly added `srcs` rules.
 

third_party/py/abseil/BUILD

@@ -1,3 +1,7 @@
+# Needed for #9006. Consider replacing label with @rules_python as per #9029 if
+# merging from upstream becomes an issue.
+load("//tools/python:private/defs.bzl", "py_library")
+
 licenses(["notice"])
 
 filegroup(

third_party/py/concurrent/BUILD

@@ -1,3 +1,7 @@
+# Needed for #9006. Consider replacing label with @rules_python as per #9029 if
+# merging from upstream becomes an issue.
+load("//tools/python:private/defs.bzl", "py_library")
+
 licenses(["notice"])
 
 filegroup(

third_party/py/gflags/BUILD

@@ -1,3 +1,7 @@
+# Needed for #9006. Consider replacing label with @rules_python as per #9029 if
+# merging from upstream becomes an issue.
+load("//tools/python:private/defs.bzl", "py_library")
+
 licenses(["notice"])
 
 filegroup(

third_party/py/mock/BUILD

@@ -1,3 +1,7 @@
+# Needed for #9006. Consider replacing label with @rules_python as per #9029 if
+# merging from upstream becomes an issue.
+load("//tools/python:private/defs.bzl", "py_library")
+
 licenses(["notice"])
 
 filegroup(

third_party/py/six/BUILD

@@ -1,3 +1,8 @@
+# Needed for #9006. We can't easily replace this label with a @rules_python
+# label (#9029) because this file is used in both @io_bazel and @bazel_tools,
+# the latter of which can't reference other repos.
+load("//tools/python:private/defs.bzl", "py_library")
+
 licenses(["notice"])
 
 filegroup(