Add Python protobuf to Bazel third_party and move all protobuf targets into
//third_party/protobuf.

--
MOS_MIGRATED_REVID=117336377
diff --git a/src/main/protobuf/BUILD b/src/main/protobuf/BUILD
index 5d275f3..979d754 100644
--- a/src/main/protobuf/BUILD
+++ b/src/main/protobuf/BUILD
@@ -1,6 +1,7 @@
 package(default_visibility = ["//visibility:public"])
 
 load("//tools/build_rules:genproto.bzl", "java_proto_library")
+load("@bazel_tools//third_party/protobuf:protobuf.bzl", "py_proto_library")
 
 FILES = [
     "build",
@@ -26,6 +27,13 @@
     src = s + ".proto",
 ) for s in FILES]
 
+py_proto_library(
+    name = "build_pb_py",
+    srcs = ["build.proto"],
+    default_runtime = "@bazel_tools//third_party/protobuf:protobuf_python",
+    protoc = "@bazel_tools//third_party/protobuf:protoc",
+)
+
 filegroup(
     name = "srcs",
     srcs = glob(["**"]),