proto-loader not populating file descriptor dependency list

### 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**
```protobuf
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**
```protobuf
syntax = "proto3";
package vendor;

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

this then inspected using the following code:
```ts
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](https://github.com/grpc/grpc-node/blob/cfa80720995d39e50b08d33efbd90e8a93a35a57/packages/grpc-reflection/src/implementations/reflection-v1.ts#L108)