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.

generate_code が read_protobuf_type と read_protobuf_service にコンテキストを渡す方法を追跡し、次に ProtoContentBase.proto_file と OutputTempate を調べます。複数ファイルのパッケージが各メッセージおよびフィールドのソース記述子をどのように選択するかを確認します。各要素を定義しているファイルから生成コメントが取得され、常に最初の入力ファイルから取得される状態でなければ完了とはみなしません。

索引モデルが issue の本文から書いたものです。