Deprecates the `repository` attribute in favor of the more general
`repositories` which expects a dict of the form

```python
{
  "@some_label": "some_name",
  "@some_other_label": "some_other_name",
}
```

and generating the `NIX_PATH`
`some_name=path(@some_label):some_other_name=path(@some_other_label)`

Make the `repositories` argument of the form `{ path_element_name: label
}` instead of `{ label: path_element_name }`