This reverts commit 7592213df29066cd357eaa4fffe4a19ed3bae189.

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.

We'll need this for compositing because we need to switch out the sky.Canvas
when we switch compositing layers.