Unexpected warnings from protoc-gen-gogo

generate.sh

# Install protoc
# Left as exercise to reader

# Install protoc-gen-gogo and friends
go get github.com/gogo/protobuf/proto
go get github.com/gogo/protobuf/jsonpb
go get github.com/gogo/protobuf/protoc-gen-gogo
go get github.com/gogo/protobuf/gogoproto

# Generate level3 file
protoc ./level3/level3.proto --gogo_out=./level3/ -I./level3/ -I$GOPATH/src

### Sample output:
# 2016/10/04 14:26:13 protoc-gen-gogo: WARNING: failed finding publicly imported dependency for .level1.FirstLevelMessage, used in level3.proto                           
# 2016/10/04 14:26:13 protoc-gen-gogo: WARNING: failed finding publicly imported dependency for .level1.FirstLevelMessage, used in level3.proto

level1.proto

syntax = "proto3";

package level1;

message FirstLevelMessage {
  int32 data = 1;
}

level2.proto

syntax = "proto3";

package level2;

import "github.com/myrepo/level1/level1.proto";

message SecondLevelMessage {
  level1.FirstLevelMessage data = 1;
}

level3.proto

syntax = "proto3";

package level3;

import "github.com/myrepo/level2/level2.proto";
import "github.com/gogo/protobuf/gogoproto/gogo.proto";

// Note that THIS is required for the warnings to appear
option (gogoproto.populate_all) = true;

message LevelThreeMessage {
  level2.SecondLevelMessage msg = 1;
}