protoc-gen-go: add test for use of publicly imported symbol

Test the following case:

File 1 defines a symbol S.
File 2 publicly imports file 1.
File 3 imports file 2 and uses S.

All files in different Go packages: P1, P2, P3.

Note that the .pb.go for file 3 could import P1 or P2, since the symbol
is defined in both.

protoc-gen-go/testdata/import_public/importing/importing.pb.go

@@ -0,0 +1,84 @@
+// Code generated by protoc-gen-go. DO NOT EDIT.
+// source: import_public/importing/importing.proto
+
+package importing
+
+import (
+	fmt "fmt"
+	proto "github.com/golang/protobuf/proto"
+	import_public "github.com/golang/protobuf/protoc-gen-go/testdata/import_public"
+	math "math"
+)
+
+// Reference imports to suppress errors if they are not otherwise used.
+var _ = proto.Marshal
+var _ = fmt.Errorf
+var _ = math.Inf
+
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the proto package it is being compiled against.
+// A compilation error at this line likely means your copy of the
+// proto package needs to be updated.
+const _ = proto.ProtoPackageIsVersion2 // please upgrade the proto package
+
+type M struct {
+	// Message type defined in a file publicly imported by a file we import.
+	M                    *import_public.M `protobuf:"bytes,1,opt,name=m" json:"m,omitempty"`
+	XXX_NoUnkeyedLiteral struct{}         `json:"-"`
+	XXX_unrecognized     []byte           `json:"-"`
+	XXX_sizecache        int32            `json:"-"`
+}
+
+func (m *M) Reset()         { *m = M{} }
+func (m *M) String() string { return proto.CompactTextString(m) }
+func (*M) ProtoMessage()    {}
+func (*M) Descriptor() ([]byte, []int) {
+	return fileDescriptor_36b835b3b8f6171a, []int{0}
+}
+
+func (m *M) XXX_Unmarshal(b []byte) error {
+	return xxx_messageInfo_M.Unmarshal(m, b)
+}
+func (m *M) XXX_Marshal(b []byte, deterministic bool) ([]byte, error) {
+	return xxx_messageInfo_M.Marshal(b, m, deterministic)
+}
+func (m *M) XXX_Merge(src proto.Message) {
+	xxx_messageInfo_M.Merge(m, src)
+}
+func (m *M) XXX_Size() int {
+	return xxx_messageInfo_M.Size(m)
+}
+func (m *M) XXX_DiscardUnknown() {
+	xxx_messageInfo_M.DiscardUnknown(m)
+}
+
+var xxx_messageInfo_M proto.InternalMessageInfo
+
+func (m *M) GetM() *import_public.M {
+	if m != nil {
+		return m.M
+	}
+	return nil
+}
+
+func init() {
+	proto.RegisterType((*M)(nil), "goproto.test.import_public.importing.M")
+}
+
+func init() {
+	proto.RegisterFile("import_public/importing/importing.proto", fileDescriptor_36b835b3b8f6171a)
+}
+
+var fileDescriptor_36b835b3b8f6171a = []byte{
+	// 153 bytes of a gzipped FileDescriptorProto
+	0x1f, 0x8b, 0x08, 0x00, 0x00, 0x00, 0x00, 0x00, 0x02, 0xff, 0xe2, 0x52, 0xcf, 0xcc, 0x2d, 0xc8,
+	0x2f, 0x2a, 0x89, 0x2f, 0x28, 0x4d, 0xca, 0xc9, 0x4c, 0xd6, 0x87, 0xf0, 0x32, 0xf3, 0xd2, 0x11,
+	0x2c, 0xbd, 0x82, 0xa2, 0xfc, 0x92, 0x7c, 0x21, 0x95, 0xf4, 0x7c, 0x30, 0x43, 0xaf, 0x24, 0xb5,
+	0xb8, 0x44, 0x0f, 0x45, 0x97, 0x1e, 0x5c, 0xad, 0x94, 0x28, 0xaa, 0x71, 0x89, 0x10, 0xcd, 0x4a,
+	0x26, 0x5c, 0x8c, 0xbe, 0x42, 0xfa, 0x5c, 0x8c, 0xb9, 0x12, 0x8c, 0x0a, 0x8c, 0x1a, 0xdc, 0x46,
+	0x8a, 0x7a, 0x78, 0x4c, 0x2b, 0x2e, 0x4d, 0xd2, 0xf3, 0x0d, 0x62, 0xcc, 0x75, 0xf2, 0x8e, 0xf2,
+	0x4c, 0xcf, 0x2c, 0xc9, 0x28, 0x4d, 0xd2, 0x4b, 0xce, 0xcf, 0xd5, 0x4f, 0xcf, 0xcf, 0x49, 0xcc,
+	0x4b, 0xd7, 0x07, 0x6b, 0x4b, 0x2a, 0x4d, 0x83, 0x30, 0x92, 0x75, 0xd3, 0x53, 0xf3, 0x74, 0xd3,
+	0xf3, 0xf5, 0x41, 0xe6, 0xa4, 0x24, 0x96, 0x24, 0xea, 0xe3, 0xf0, 0x0f, 0x20, 0x00, 0x00, 0xff,
+	0xff, 0xd8, 0x7e, 0x58, 0x1c, 0xe9, 0x00, 0x00, 0x00,
+}

protoc-gen-go/testdata/import_public/importing/importing.proto

@@ -0,0 +1,43 @@
+// Go support for Protocol Buffers - Google's data interchange format
+//
+// Copyright 2018 The Go Authors.  All rights reserved.
+// https://github.com/golang/protobuf
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+syntax = "proto2";
+
+package goproto.test.import_public.importing;
+
+option go_package = "github.com/golang/protobuf/protoc-gen-go/testdata/import_public/importing";
+
+import "import_public/a.proto";
+
+message M {
+  // Message type defined in a file publicly imported by a file we import.
+  optional goproto.test.import_public.sub.M m = 1;
+}