1) Moved basic utility code into base/ directory to make it clear which code
   doesn't depend on Flutter-specific knowldge.
2) Move the CommandRunner subclasses into a runner/ directory because these
   aren't commands themselves.

This package contains WidgetTester, which is very useful when writing tests for
widgets.

Converted demo launcher example to library

Converted fitness example to library

Converted hello world example to library

Converted mine digger example to library

Converted stocks example to library