Support for multiple NBs with same `default_exp`

From the base `nbdev_template` you can duplicate `00_core.ipynb` to make `01_other_core.ipynb` and change the name of the `foo` function declared in this duplicate notebook to `bar`. Then you have two notebooks with the same `default_exp` exporting distinct functions.

The `core.py` generated from then executing `nbdev_export` ignores the `foo` function from `00_core.ipnyb`. And when you run `nbdev_preview`, the sidebar links twice to the same `http://localhost:3000/core.html`.

nbdev1 handles this more cleanly. I think it's an important feature as some awkward decisions around documentationare forced otherwise.

Reproduce the case from the base nbdev_template by duplicating 00_core.ipynb, changing foo to bar, and running nbdev_export and nbdev_preview. Trace those entry points to identify why one export is omitted and why the sidebar links twice to core.html; done means both functions are exported and the notebooks receive distinct sidebar links.

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