Handle code in __init__.py files

[benjaminjkraft]

This is another pattern that is not used at KA but is common in the rest of the world. We should be able to move directories containing nontrivial __init__.py files as well as symbols defined in __init__.py. Moving the whole file gets a little weird since we need to figure out what's a symbol in __init__.py vs. a separate module. (This thus also has interactions with #20.)