add importpath_aliases to go_proto_library (bazelbuild#2608)

It allows go_proto_library to accept importpath_aliases as an argument. It is needed because gazelle generates go_proto_library rules that have importpath_aliases as an argument.

Fixes bazelbuild#2358

proto/def.bzl

@@ -152,6 +152,7 @@ go_proto_library = go_rule(
         ),
         "importpath": attr.string(),
         "importmap": attr.string(),
+        "importpath_aliases": attr.string_list(),  # experimental, undocumented
         "embed": attr.label_list(providers = [GoLibrary]),
         "gc_goopts": attr.string_list(),
         "compiler": attr.label(providers = [GoProtoCompiler]),

tests/core/go_proto_library/BUILD.bazel

@@ -117,9 +117,16 @@ go_test(
     deps = [":protos_go_proto"],
 )
 
+go_test(
+    name = "protos_alias_test",
+    srcs = ["protos_alias_test.go"],
+    deps = [":protos_go_proto"],
+)
+
 go_proto_library(
     name = "protos_go_proto",
     importpath = "github.com/bazelbuild/rules_go/tests/core/go_proto_library/protos",
+    importpath_aliases = ["myalias/protos"],
     protos = [
         ":protos_a_proto",
         ":protos_b_proto",

tests/core/go_proto_library/README.rst

@@ -53,3 +53,9 @@ wkt_wrapper_test
 Checks that most of the well known types in ``//proto/wkt`` are wrappers
 for packages in ``@org_golang_google_protobuf``. The proto types should be
 type aliases.
+
+protos_alias_test
+-----------------
+
+Checks that packages generated by `go_proto_library` can be imported using one of the strings
+listed in ``importpath_aliases``.

tests/core/go_proto_library/protos_alias_test.go

@@ -0,0 +1,29 @@
+/* Copyright 2020 The Bazel Authors. All rights reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+   http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package protos_alias_test
+
+import (
+	"myalias/protos" // importing based on the alias import path
+	"testing"
+)
+
+func use(interface{}) {}
+
+func TestProtos(t *testing.T) {
+	// just make sure both types exist
+	use(protos.A{})
+	use(protos.B{})
+}