Better organize Bazel build definitions so that rules_go isn't a hard dependency

[meteorcloudy]

Currently, proto targets for different languages are defined in a BUILD file by loading the xds_proto_package macro (eg. in //xds/data/orca/v3). The bzl file that defines xds_proto_package again loads macros from rules_go:

xds/bazel/api_build_system.bzl

Line 4 in 1e77728

load("@io_bazel_rules_go//proto:def.bzl", "go_grpc_library", "go_proto_library")

This is a bad practice because it forces any other project (see grpc) which depends on any target in //xds/data/orca/v3 to fetch rules_go, even though the project doesn't depend on the go_proto_library target. This is a common pitfall with Bazel, see bazelbuild/bazel#12835

A better package structure is to split targets into different packages by language, eg

• proto_library under //xds/data/orca/v3
• go_proto_library under //xds/data/orca/v3/go
• py_proto_library under //xds/data/orca/v3/py
• cc_proto_library under //xds/data/orca/v3/cc

so that dependencies for specific languages are not fetched when the corresponding package is not needed.

To maintain backwards compatibility, you can still have alias targets in //xds/data/orca/v3that points to other language specific packages.

[meteorcloudy]

FYI @aranguyen @Wyverald