which ensures we won't have inconsistent black-vs-isort errors
going forward. We can always format by running `ufmt format .`
at the root, and check with `ufmt check .` in our CI actions.
This introduces a few new gotchas (namely attribute access and a bug with
Union[Callable]), but it also removes a whole host of pyre-fixmes and gets us
updated to a release of pyre that came out after May.
Standardize on the convention that private modules (those we don't expect people to directly import) are prefixed with an underscore. Everything under a directory/module that has an underscore is considered private, unless it is re-exported from a non-underscored module. Most things are exported from libcst directly, but there are a few things in libcst.tool, libcst.codegen and libcst.metadata that are namedspaced as such.
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.
