Incorporates the fixtures from eslint-plugin-react-hooks using a script, so that
we can easily update them in the future. For each fixture we run the compiler
with and without hooks validation first so that we know if the fixture is
expected to pass — we have some false positives and false negatives that i can
work through. For example we accidentally think that `userFetch()` is a hook,
oops. Fixtures that should pass but error, or that should error but pass, are
marked as `todo.<name>` or `todo.error.<name>`.
While i was here i added the ability to have fixtures in subdirectories for
grouping purposes.