Allow a single notebook to export to multiple .py modules

[jph00]

Use the same #|export modname syntax that was in nbdev1

[hugetim]

The directives docs suggest this issue can be closed, no?

[jph00]

Maybe you could give it a try and see if it works? I don't recall where we got to with this.

[hugetim]

It doesn't work in my simple example. The export directive silently fails, not exporting the function anywhere:
nbdev-hello-world-test.zip

[turbotimon]

@hugetim it works in my example with nbdev Version: 2.3.13

01_two.ipynb:

# | default_exp two

#| export core
def foo_core_mixin(): # is exported to core.py
    pass

#| export
def foo_two(): # is exported to two.py
    pass

However, I don't like the fact that it appears that you can't have two notebooks with the same default_exp (as it says here, it gets overwritten. However, i can't find anything in the docs about this)