Missing module name in _twirp generated code

# When I generate code from two proto files
```proto
// data.proto
syntax = "proto3";
import "ok.proto";

service Data {
rpc create(SetDataRequest) returns (ok.Response);
}

message SetDataRequest {
string uuid = 1;
}
```
```proto
// ok.proto
syntax = "proto3";
package ok;

message Response { }
```
### With one protoc execute
```sh
protoc --proto_path=. --ruby_out=. --twirp_ruby_out=. --plugin=/Users/a.zimin/go/bin/protoc-gen-twirp_ruby data.proto ok.proto
```

### I have a code with a missing module name of Response class
```ruby
# data_twirp.rb
# Code generated by protoc-gen-twirp_ruby 1.4.1, DO NOT EDIT.
# ...SKIP...
rpc :create, SetDataRequest, Response, :ruby_method => :create
# ...SKIP...
```
```ruby
module Ok
Response = ::Google::Protobuf::DescriptorPool.generated_pool.lookup("ok.Response").msgclass
end
```
### With two protoc calls
```sh
protoc --proto_path=. --ruby_out=. --twirp_ruby_out=. --plugin=/Users/a.zimin/go/bin/protoc-gen-twirp_ruby data.proto
protoc --proto_path=. --ruby_out=. --twirp_ruby_out=. --plugin=/Users/a.zimin/go/bin/protoc-gen-twirp_ruby ok.proto
```

### Generated code is correct
```ruby
# data_twirp.rb
# ...SKIP...
rpc :create, SetDataRequest, Ok::Response, :ruby_method => :create
# ...SKIP...
```

Reproduce the issue with data.proto and ok.proto using the two protoc command variants, then compare data_twirp.rb with the separately generated output. Trace the generator behavior that determines the RPC response type; done means the combined invocation emits Ok::Response just like the separate invocations.

Written by the indexing model from the issue text.