Add go_proto_library plus generated files for "go build". (#72)

Generating Go code like this matches the pattern precedented by https://github.com/google/go-genproto for Google APIs.

Generated files are placed in:
- v2/ for remote execution, matching the style used by go-genproto for versioned APIs, e.g. https://github.com/google/go-genproto/blob/master/googleapis/appengine/v1/appengine.pb.go#L4
- semver/ for semver library, matching the style used by go-genproto for unversioned APIs, e.g. https://github.com/google/go-genproto/blob/master/googleapis/api/distribution/distribution.pb.go#L4

Unlike go-genproto, we use Bazel to generate the Go code, which is much simpler, since it handles calling protoc for us. We add a pre-commit hook to generate Go code before each commit, and add it to that commit.

go.mod and go.sum generated by:

```
go mod init github.com/bazelbuild/remote-apis
go build ./...
```

I tested this by creating a Go project that imports this proto and confirmed that I can build it with Bazel and go build.

This addresses issue #65.

README.md

@@ -60,3 +60,28 @@ Bazel, you will possibly want to declare `cc_proto_library`,
 
 Other build systems will have to run protoc on the protobuf files, and link in
 the googleapis and well-known proto types, manually.
+
+### Go (for non-Bazel build systems)
+
+This repository contains the generated Go code for interacting with the API via
+gRPC. Get it with:
+
+```
+go get github.com/bazelbuild/remote-apis
+```
+
+Import it with, for example:
+
+```
+repb "github.com/bazelbuild/remote-apis/build/bazel/remote/execution/v2"
+```
+
+## Development
+
+Enable the git hooks to automatically generate Go proto code on commit:
+
+```
+git config core.hooksPath hooks/
+```
+
+This is a local setting, so applies only to this repository.

WORKSPACE

@@ -54,3 +54,25 @@ bind(
     name = "grpc_lib",
     actual = "@com_github_grpc_grpc//:grpc++",
 )
+
+# We have to import zlib directly ourselves, because protobuf_deps.bzl isn't
+# part of the protobuf release yet
+# (https://github.com/protocolbuffers/protobuf/issues/5918). This should be
+# fixed in 3.8.0.
+http_archive(
+    name = "net_zlib",
+    build_file = "@com_google_protobuf//:third_party/zlib.BUILD",
+    sha256 = "6d4d6640ca3121620995ee255945161821218752b551a1a180f4215f7d124d45",
+    strip_prefix = "zlib-1.2.11",
+    urls = ["https://zlib.net/zlib-1.2.11.tar.gz"],
+)
+
+# Pull in go rules.
+http_archive(
+    name = "io_bazel_rules_go",
+    sha256 = "86ae934bd4c43b99893fc64be9d9fc684b81461581df7ea8fc291c816f5ee8c5",
+    urls = ["https://github.com/bazelbuild/rules_go/releases/download/0.18.3/rules_go-0.18.3.tar.gz"],
+)
+load("@io_bazel_rules_go//go:deps.bzl", "go_rules_dependencies", "go_register_toolchains")
+go_rules_dependencies()
+go_register_toolchains()

build/bazel/remote/execution/v2/BUILD

@@ -1,5 +1,7 @@
 package(default_visibility = ["//visibility:public"])
 load("@com_github_grpc_grpc//bazel:cc_grpc_library.bzl", "cc_grpc_library")
+load("@io_bazel_rules_go//go:def.bzl", "go_library")
+load("@io_bazel_rules_go//proto:def.bzl", "go_proto_library")
 
 licenses(["notice"])
 
@@ -37,3 +39,21 @@ cc_grpc_library(
     use_external = False,
 )
 
+go_proto_library(
+    name = "remote_execution_go_proto",
+    compilers = ["@io_bazel_rules_go//proto:go_grpc"],
+    importpath = "github.com/bazelbuild/remote-apis/build/bazel/remote/execution/v2",
+    proto = ":remote_execution_proto",
+    deps = [
+        "//build/bazel/semver:go_default_library",
+        "@go_googleapis//google/api:annotations_go_proto",
+        "@go_googleapis//google/longrunning:longrunning_go_proto",
+        "@go_googleapis//google/rpc:status_go_proto",
+    ],
+)
+
+go_library(
+    name = "go_default_library",
+    embed = [":remote_execution_go_proto"],
+    importpath = "github.com/bazelbuild/remote-apis/build/bazel/remote/execution/v2",
+)