Intelligently rerun `nbdev_export` in `nbdev_merge` for more automated conflict resolution

I've had a few instances now where notebook conflicts were resolved but where the modules and/or `_modidx` were still in a conflicted state. We might be able to run `nbdev_export` in the merge driver to fix that. I haven't yet thought about how complicated the logic would need to be.

Start by examining the nbdev_merge and nbdev_export entry points and how the merge driver handles resolved notebook conflicts. Reproduce a conflict where notebooks are resolved but modules or _modidx remain conflicted; done means the merge process intelligently reruns nbdev_export when needed and leaves those generated files consistent.

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