Aggressively apply the const lint.

The new name follows the pattern of the name suggesting which layout
protocol the parent expects the child to speak.

Fixes #8664

Rename all the "2" classes related to scrolling to remove the "2". Now that the
old scrolling code is gone, we don't need to use the suffix.

Previously the "left" padding was applied on the right when hit testing
a reversed vertical list view.

This structure makes it easier to customize the viewport used in a Scrollable.
Now Scrollable2 doesn't commit to using slivers.