Compiler option to specify Python package prefix
I'm packaging an application that will include some protocol parsing logic using KS. I made fairly extensive use of `meta/imports` to minimize spec duplication. When compiling to Python however, the resulting imports end up not being scoped to any package, as in the example below for `MyImportedType`:

```Python
...
from my_imported_type import MyImportedType
class MyBaseType(KaitaiStruct):
    def __init__(self, _io, _parent=None, _root=None):
...
```

The becomes an issue when running this application, as that module does not exist anywhere in `$PYTHONPATH` but instead gets installed as part of my application's Python package. It should be something more like `from application.protocol.my_imported_type import MyImportedType`, or perhaps a relative import.

I'm not sure if this is as simple as providing a flag to the compiler that will prepend the imports with the given string:
`kaitai-struct-compiler --target python --python-package "application.protocol" --import-path ksy/ ksy/*`