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.

Trace how generate_code passes context into read_protobuf_type and read_protobuf_service, then inspect ProtoContentBase.proto_file and OutputTempate. Confirm how a multi-file package selects the source descriptor for each message and field. Done means generated comments come from the file defining each item rather than always from the first input file.

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