Move the animation libraries into src/animation and change importers to use
package:sky/animation.dart. Also, move scheduler.dart into the animation
library so that the animation library can be self-contained.
Code outside of package:sky should import this code using
package:sky/rendering.dart
package:sky/widgets.dart
Moving this code into the "src" directory is a convention that signifies that
and it cleans up the generated dartdoc because the libraries in the src
directory aren't included in the generated documentation. Instead, the classes
are documented in the widgets.dart and rendering.dart libraries.
