Importing well known types

Hello,

I'm not sure how to work with well known types.

Having a proto file like this:

```
syntax = "proto3";

package simple;

import "google/protobuf/empty.proto";

service SimpleSender {
rpc Send(SendParams) returns (google.protobuf.Empty) {}
}

message SendParams {
string body = 1;
}
```

I get output like this:

```python
# Generated by the protocol buffer compiler. DO NOT EDIT!
# sources: Simple.proto
# plugin: python-betterproto
from dataclasses import dataclass

import betterproto
import grpclib

from .google import protobuf


@dataclass
class SendParams(betterproto.Message):
body: str = betterproto.string_field(1)


class SimpleSenderStub(betterproto.ServiceStub):
async def send(self, *, body: str = "") -> protobuf.Empty:
request = SendParams()
request.body = body

return await self._unary_unary(
"/simple.SimpleSender/Send", request, protobuf.Empty,
)
```

There is ofcourse no `google` package. I tried to generate code from proto files included in `grpc_tools` but I'm not able to get any python code.

What is the best way to work with well known types?