[dart] First stab add correct Dart lib layout

Initial draft for new Dart library layout that closely follows Darts best practices.

• move everything under a src folder
• add new top level file called $pubName.dart that exports all other files
• users now only require one import 'package:$pubName/$pubName.dart';

Most of the changes are just moved files and changed imports.

Looking for feedback @agilob @josh-burton

I assume changing the file layout and requiring users to change their imports is considered a breaking change? @wing328