Set --grpc-gateway:. from --grpc-gateway_out:$GOPATH/src

grpc-gateway v1.3 doesn't contain grpc-ecosystem/grpc-gateway#462
which fixes a bug that prevents grpc-gateway from fully respecting
the go_path option in proto files.

gen.go

@@ -17,4 +17,4 @@ package trillian
 //go:generate protoc -I=. -I=$GOPATH/src -I=$GOPATH/src/github.com/googleapis/googleapis --go_out=plugins=grpc:$GOPATH/src trillian_log_api.proto trillian_map_api.proto trillian_admin_api.proto trillian.proto
 //go:generate protoc -I=. --go_out=:$GOPATH/src crypto/sigpb/sigpb.proto
 //go:generate protoc -I=. --go_out=:$GOPATH/src crypto/keyspb/keyspb.proto
-//go:generate protoc -I=. -I=$GOPATH/src -I=$GOPATH/src/github.com/google/trillian/vendor/github.com/grpc-ecosystem/grpc-gateway/third_party/googleapis -I=$GOPATH/src/github.com/googleapis/googleapis/ --grpc-gateway_out=logtostderr=true:$GOPATH/src trillian_log_api.proto trillian_map_api.proto trillian_admin_api.proto trillian.proto
+//go:generate protoc -I=. -I=$GOPATH/src -I=$GOPATH/src/github.com/google/trillian/vendor/github.com/grpc-ecosystem/grpc-gateway/third_party/googleapis -I=$GOPATH/src/github.com/googleapis/googleapis/ --grpc-gateway_out=logtostderr=true:. trillian_log_api.proto trillian_map_api.proto trillian_admin_api.proto trillian.proto