api_go_proto_library is used in an ad-hoc fashion. It's not part of the api_proto_library macro, and is often neglected when protos are shuffled.

This issue is a proposal to organize the protos as follows:

• there should be only one go_proto_library per directory in xDS. This is aligned with normal Golang practices of Go package == proto package (e.g. https://github.com/googleapis/go-genproto/tree/master/googleapis/api/annotations);
• there should be a source proto_library per-directory with all protos as sources;
• all other go_proto_library should be removed.

This seems like a middle-ground due to various Bazel issues with golang:

• you cannot have two proto libraries as inputs to a Go proto library;
• you cannot have two Go proto libraries for the same proto.

This is also aligned with go-control-plane structure. Having one package per proto is onerous for Go developers.

Ref envoyproxy/go-control-plane#213