ProtoContentBase.proto_file is always the first file of a package

I have a large protobuf package whose messages, etc. are defined over multiple files. However, `ProtoContentBase.proto_file` always refers to the *first* file of the package passed to `protoc`. As a result, when I run `protoc` the comments attached to each class and field are from the first file.

I hacked a solution to this by adding `proto_input_file` to `ProtoContentBase` and passing this as an argument all the way from `generate_code` through `read_protobuf_type`/`read_protobuf_service`, then adding:

- To `OutputTempate`:
```python
@property
def input_files_dict(self) -> Dict[str, FileDescriptorProto]:
"""Dictionary mapping filenames to their FileDescriptorProto objects

Returns
-------
Dict[str, FileDescriptorProto]
Dictionary mapping filenames to their FileDescriptorProto objects
"""
return {f.name: f for f in self.input_files}
```

- to `ProtoContentBase`
```python
@property
def proto_file(self) -> FieldDescriptorProto:
template = self.output_file
return template.input_files_dict[self.proto_input_file]
```

I'd be delighted to contribute a proper fix and open a PR for it, but it'd be good to hear if there's a better approach to solving the problem.

Rastrea cómo generate_code pasa el contexto a read_protobuf_type y read_protobuf_service; después, inspecciona ProtoContentBase.proto_file y OutputTempate. Confirma cómo un paquete con varios archivos selecciona el descriptor de origen para cada mensaje y campo. Se considera terminado cuando los comentarios generados provienen del archivo que define cada elemento, en lugar de provenir siempre del primer archivo de entrada.

Escrito por el modelo de indexación a partir del texto del issue.