Issue importing structures between files in subpackages

Given a file like...

```
import "badger/mushroom";

package badger.snake;

message Snake {
badger.mushroom.Mushroom mushroom = ...;
}
```

This results in compiling a `badger/snake.py` which includes the line `from .badger import mushroom` instead of `from . import mushroom`.

Reproduce the example with the badger.mushroom import and badger.snake package, then inspect the generated badger/snake.py. Trace how the compiler resolves imports between subpackages; done means the generated file uses `from . import mushroom` and still compiles correctly.

Written by the indexing model from the issue text.