Do not allow deep imports from feature folders

[JoosepAlviste]

Only from index files that are the "public" API for the feature. We can use an ESLint rule to check this.

However, we'll need to figure out how to handle schema file imports since they have side effects and should probably not be imported into the index files.

https://github.com/alan2207/bulletproof-react/blob/master/docs/project-structure.md