Stack trace on create-generators if hyphen in the story generators module name.

`story_generators_module: ssg-omop` causes `create-generators` to fail with a stack trace.

The solutions are: just don't do that (raise an error saying please don't do that), or:

```python
import importlib
ssg_omop = importlib.import_module("ssg-omop")
```

But this is a difficult solution because all references to `ssg-omop` will need to be changed to `ssg_omop`.
