Nested packages causes invalid imports

Compilation of proto files using 2.0.0b3
```sh
protoc -I . --python_betterproto_out=prototest base.proto example.proto
```
```proto
# base.proto
syntax = "proto3";

package Foo;

message Bar {
string value = 1;
}
```
```proto
# example.proto
syntax = "proto3";

import "base.proto";

package Foo.Example;

message Example {
Bar value = 1;
}
```
returns `Foo/__init__.py`
```python
# 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`
```python
# 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__`.