Currently we build the Sass import graph when the CLI watcher is
started. However the graph is not update updated when files are
added or deleted. The latest is a big deal but the former results
in new files not triggered rebuilds when they're changed. The only
way to currently resolve this is to restart the CLI watcher.

This patch rebuilds the Sass import graph when files are added or
deleted to the watch always works as expected.

Fixes #1538