Nested packages causes invalid imports

Compilation of proto files using 2.0.0b3

protoc -I . --python_betterproto_out=prototest base.proto example.proto

# base.proto
syntax = "proto3";

package Foo;

message Bar {
    string value = 1;
}

# example.proto
syntax = "proto3";

import "base.proto";

package Foo.Example;

message Example {
    Bar value = 1;
}

returns Foo/__init__.py

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

import betterproto
from betterproto.grpc.grpclib_server import ServiceBase


@dataclass(eq=False, repr=False)
class Bar(betterproto.Message):
    value: str = betterproto.string_field(1)

Foo/Example/__init__.py

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

import betterproto
from betterproto.grpc.grpclib_server import ServiceBase


@dataclass(eq=False, repr=False)
class Example(betterproto.Message):
    value: "__FooBar__" = betterproto.message_field(1)


from ... import FooBar as __FooBar__

The last import in Foo/Example/__init__.py is invalid, since there is no FooBar module. It probably should have been from Foo import Bar as __FooBar__.

Reproduce the issue with the shown protoc command and inspect the generated Foo/init.py and Foo/Example/init.py files. Trace how the imported Bar message is resolved during generation; done means the nested package output contains a valid import for Bar and the generated files compile successfully.