Go through and clean up where we export things from in the top level
directories. This is the first pass at cleaning up the public API. In
this diff I clean up all top-level exports that were coming out of .py
files and instead export them from libcst's base module itself.
I also didn't export a few of the things defined in newly underscored
files because they were only used internally.
Strings are safe to use with a word operator in many cases, so this
fixes it to support that.
This was causing some parser errors in
`commerce/shopping_recommendations_service_shots.py` due to some whacky
f-strings. We weren't encountering those errors before, because of other
f-string parsing issues that we were hitting first.
