Fix typing and datetime imports not being present for service method type annotations

[leenr]

Example messages.proto:

syntax = "proto3";

import "google/protobuf/duration.proto";
import "google/protobuf/timestamp.proto";

package things.messages;

message DoThingRequest {
  string name = 1;
  repeated string comments = 2;
  google.protobuf.Timestamp when = 3;
  google.protobuf.Duration duration = 4;
}

message DoThingResponse {
  repeated string names = 1;
}

Example service.proto:

syntax = "proto3";

import "messages.proto";

package things.service;

service Test {
  rpc DoThing (things.messages.DoThingRequest) returns (things.messages.DoThingResponse);
}

---

Using current master (1d54ef8):

$ protoc --python_betterproto_out=.local/servicemethod_imports_issue/output_master_1d54ef8 --proto_path=.local/servicemethod_imports_issue/proto .local/servicemethod_imports_issue/proto/*.proto
Writing __init__.py
Writing things/__init__.py
Writing things/messages/__init__.py
Writing things/service/__init__.py

$ head -n20 .local/servicemethod_imports_issue/output_master_1d54ef8/things/service/__init__.py
# Generated by the protocol buffer compiler.  DO NOT EDIT!
# sources: service.proto
# plugin: python-betterproto
from dataclasses import dataclass
from typing import Dict, Optional

import betterproto
from betterproto.grpc.grpclib_server import ServiceBase
import grpclib


class TestStub(betterproto.ServiceStub):
    async def do_thing(
        self,
        *,
        name: str = "",
        comments: Optional[List[str]] = None,
        when: datetime = None,
        duration: timedelta = None,
    ) -> "_messages__.DoThingResponse":

Notice List, datetime and timedelta being used in type annotation, but not listed in imports.

---

Using version from this PR:

$ protoc --python_betterproto_out=.local/servicemethod_imports_issue/output_fix_b854d38 --proto_path=.local/servicemethod_imports_issue/proto .local/servicemethod_imports_issue/proto/*.proto
Writing __init__.py
Writing things/__init__.py
Writing things/messages/__init__.py
Writing things/service/__init__.py

$ head -n21 .local/servicemethod_imports_issue/output_fix_b854d38/things/service/__init__.py
# Generated by the protocol buffer compiler.  DO NOT EDIT!
# sources: service.proto
# plugin: python-betterproto
from dataclasses import dataclass
from datetime import datetime, timedelta
from typing import Dict, List, Optional

import betterproto
from betterproto.grpc.grpclib_server import ServiceBase
import grpclib


class TestStub(betterproto.ServiceStub):
    async def do_thing(
        self,
        *,
        name: str = "",
        comments: Optional[List[str]] = None,
        when: datetime = None,
        duration: timedelta = None,
    ) -> "_messages__.DoThingResponse":

---

I've also implemented a copy of test's "things service" (service), separated in two protobuf packages: one with messages, other with service definition, and added Duration and Timestamp fields to DoThingRequest message.
It fails on current master (1d54ef8), but succeeds with the changes from this PR.

[nat-n]

Thanks for the fix :)

[nat-n]

Do I understand correctly that this line is the actual fix?

[leenr]

Yes, along with the other changes.

I've deleted adding "Optional" import two lines above because there already is the code to add it in the FieldCompiler class:

if "Optional[" in annotation:
    imports.add("Optional")

[huan]

Looking forward to having this PR merged!

It will fix my wechaty/grpc#120