feat(proto): Add protobuf & gRPC dependencies via external

Moves the Protobuf & gRPC dependencies to a separate file which uses
buildGo.external to build the dependencies.

The versions are pinned at master of 2019-11-26.

buildGo.nix

@@ -123,18 +123,8 @@ let
       paths = [ gopathSrc gopathPkg ];
     }) // { goDeps = uniqueDeps; };
 
-  # Protobuf & gRPC integration requires these dependencies:
-  proto-go-src = fetchFromGitHub {
-    owner = "golang";
-    repo = "protobuf";
-    rev = "ed6926b37a637426117ccab59282c3839528a700";
-    sha256 = "0fynqrim022x9xi2bivkw19npbz4316v4yr7mb677s9s36z4dc4h";
-  };
-
-  goProto = external {
-    path = "github.com/golang/protobuf";
-    src = proto-go-src;
-    deps = [];
+  protoLibs = import ./proto.nix {
+    inherit external;
   };
 in {
   # Only the high-level builder functions are exposed

proto.nix

@@ -0,0 +1,72 @@
+# Copyright 2019 Google LLC.
+# SPDX-License-Identifier: Apache-2.0
+#
+# This file provides derivations for the dependencies of a gRPC
+# service in Go.
+
+{ external }:
+
+let
+  inherit (builtins) fetchGit map;
+in rec {
+  goProto = external {
+    path = "github.com/golang/protobuf";
+    src = fetchGit {
+      url = "https://github.com/golang/protobuf";
+      rev = "ed6926b37a637426117ccab59282c3839528a700";
+    };
+  };
+
+  xnet = external {
+    path = "golang.org/x/net";
+    srcOnly = true;
+    deps = [ xtext ];
+    src = fetchGit {
+      url = "https://go.googlesource.com/net";
+      rev = "ffdde105785063a81acd95bdf89ea53f6e0aac2d";
+    };
+  };
+
+  xsys = external {
+    path = "golang.org/x/sys";
+    srcOnly = true;
+    src = fetchGit {
+      url = "https://go.googlesource.com/sys";
+      rev = "bd437916bb0eb726b873ee8e9b2dcf212d32e2fd";
+    };
+  };
+
+  xtext = external {
+    path = "golang.org/x/text";
+    srcOnly = true;
+    src = fetchGit {
+      url = "https://go.googlesource.com/text";
+      rev = "cbf43d21aaebfdfeb81d91a5f444d13a3046e686";
+    };
+  };
+
+  genproto = external {
+    path = "google.golang.org/genproto";
+    srcOnly = true;
+    src = fetchGit {
+      url = "https://github.com/google/go-genproto";
+      rev = "83cc0476cb11ea0da33dacd4c6354ab192de6fe6";
+    };
+  };
+
+  goGrpc = external {
+    path = "google.golang.org/grpc";
+    deps = [ goProto xnet xsys genproto ];
+
+    src = fetchGit {
+      url = "https://github.com/grpc/grpc-go";
+      rev = "d8e3da36ac481ef00e510ca119f6b68177713689";
+    };
+
+    targets = [
+      "."
+      "codes"
+      "status"
+    ];
+  };
+}