Relative imports

Nadie ha tomado este issue todavía.

Certainly I had the same issue with the original protoc compiler, but generated relative imports are complaining. I have a bunch of protos in the same directory.

I'm using the cmd line to generate the dataclasses:

```python -m grpc_tools.protoc -I C:\dir\libroot --python_betterproto_out=src\libs C:\dir\libroot\Protos\myproto.proto```

This generates the proto files in the expected location `src\libs\packagename\myproto.py`, with the following imports, which don't work:

```from .packagename import common```

However (in my use case) this needs to be:

```from . import common```

But perhaps I've got the wrong end of the stick!

Thanks.

Reproduce el problema con el comando indicado python -m grpc_tools.protoc e inspecciona los imports de src\libs\packagename\myproto.py generado para protos que comparten un directorio. Se considera terminado cuando los imports relativos generados se resuelvan correctamente para esta estructura sin ediciones manuales.

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