Broken build due to missing protobuf:py_proto_library rule

[veblush]

gRPC recently found out that cncf/xds repo won't work with the HEAD version of protobuf because protobuf:py_proto_library is removed (technically renamed) by protocolbuffers/protobuf#10132. Therefore, build ends up with the following error.

ERROR: Traceback (most recent call last):
        File "/usr/local/google/home/veblush/.cache/bazel/_bazel_veblush/c4652c20fd8d5880d194bf82693e4fee/external/com_github_cncf_udpa/bazel/api_build_system.bzl", line 2, column 66, in <toplevel>
                load("@com_google_protobuf//:protobuf.bzl", _py_proto_library = "py_proto_library")
Error: file '@com_google_protobuf//:protobuf.bzl' does not contain symbol 'py_proto_library'
ERROR: /usr/local/google/home/veblush/git/grpc/BUILD:7688:34: error loading package '@com_github_cncf_udpa//xds/type/v3': Extension file 'bazel/api_build_system.bzl' has errors and referenced by '//:xds_type_upbdefs'

They say that protobuf:py_proto_library is an internal rule so you may want to move away from it.

[sergiitk]

@veblush this should be resolved now by #74.