proto-loader not populating file descriptor dependency list

[jtimmons]

Problem description

The *Definition structures that proto-loader returns each have a fileDescriptorProtos field which include the files needed to resolve each type. The file descriptor includes a "dependency list" of files that they require but this is currently always returning nothing

This was discussed in #79

Reproduction steps

This can be reproduced using the following two proto file definitions:
sample.proto

syntax = "proto3";
package sample;
import 'vendor.proto';

service SampleService {
  rpc Hello (HelloRequest) returns (HelloResponse) {}
}

message HelloRequest {
  string hello = 1;
}

message HelloResponse {
  vendor.HelloStatus status = 2;
}

vendor.proto

syntax = "proto3";
package vendor;

enum HelloStatus {
  HELLO = 1;
  WORLD = 2;
}

this then inspected using the following code:

import { FileDescriptorProto } from 'google-protobuf/google/protobuf/descriptor_pb';
import * as protoLoader from '@grpc/proto-loader';

const root = protoLoader.loadSync('sample.proto', { includeDirs: [__dirname] });

Object.values(root).forEach(({ fileDescriptorProtos }) => {
  if (Array.isArray(fileDescriptorProtos)) {
    fileDescriptorProtos.forEach((bin) => {
      const proto = FileDescriptorProto.deserializeBinary(bin);
      console.log(`File: ${proto.getName()}\tDependencies: ${proto.getDependencyList()}`);
      /* output:
File: sample.proto      Dependencies: 
File: vendor.proto      Dependencies: 
File: sample.proto      Dependencies: 
File: vendor.proto      Dependencies: 
File: sample.proto      Dependencies: 
File: vendor.proto      Dependencies: 
      */
    });
  }
});

we would expect that sample.proto would include vendor.proto in its dependency list due to the import and usage of it in the HelloResponse field

Environment

• OS name, version and architecture: [e.g. Linux Ubuntu 18.04 amd64]: Mac OS 13.4.1
• Node version [e.g. 8.10.0]: v20.8.0
• Node installation method [e.g. nvm]: nvm
• If applicable, compiler version [e.g. clang 3.8.0-2ubuntu4]: N/A
• Package name and version [e.g. gRPC@1.12.0]: proto-loader@0.6.9

Additional context

We are working around this in the @grpc/reflection package by manually adding those dependencies based on the proto definitions with this visitor-pattern-based logic