If users do `from flake8.api.legacy import *` we only want them to get
get_style_guide imported. The other classes are not meant to be created
by users.

We need to initialize part of the Application so we can set options
passed by the user, but we also want to delay making things, e.g.,

- Formatter
- Style Guide
- etc.

Until we have the options solidified so we don't have to do annoying
things.

This does not handle setting custom options via the parameters to
get_style_guide.

This is an emerging best practice and there is little reason to not
follow it