`nbdev_update` fails when `lib_name` is not the same as `lib_path`

The problem is here:

https://github.com/fastai/nbdev/blob/46d05d40fb8bfd1f6466f3daabc1fcf300a73fde/nbdev/sync.py#L33

It should be 

```python
 midx = import_module(f'{get_config().lib_path}._modidx') 
```