Allow modules in pytest header to be customized

With this, affiliated packages can do (in ``conftest.py``):

```
PYTEST_HEADER_MODULES['scikit-image'] = 'skimage'
```
For some reason I thought I had to do this in the past (this is basically what ``importlib`` does). But I think that was for packages that have sub-modules or something like that. Anyway, I'll rebase and fix this.