Refactor project structure to one single module?

Hi guys, as you can see I have been working on a single module project structure.
Here is the PR: https://github.com/android10/Android-CleanArchitecture/pull/103

I'm not saying that I will land it but I do want to start a discussion around it and see pros and cons for either a single module project or multi module project.

The first couple of things that came to my mind:
### Advantages:
- **Scalability:** package organization into features so working with a feature team model will fit better, specially when the number of contributors to the codebase is big.
- **Scope:** favors exposure to only necessary members (removes many public methods from feature specific classes).
- **Build System:** less overhead since building logic sits on only one project and we do not have to maintain many `build.gradle` files for each project (better performance?).
### Drawbacks:
- **Clarity:** by having 3 modules, each layer responsibility is very clear.
- **Dependency with the framework:** By having only one android module we are exposed to make mistakes and break both dependency rule and dependency on android/java framework.

A few extra eyes are very appreciated, so let's try to find the best way to keep on improving this codebase.

@spirosoik @Trikke @caseykulm I mention you guys since have been collaborating a lot. Thanks! Feel free to involve anyone here :smiley: