Prior to this, the watcher handled all the logic around recompiling
stylesheets if an upstream file was deleted or added in a way that
could affect their import resolution. This left a gap where the
stylesheet graph wouldn't be aware that a newly-added file had become
upstream dependency of an existing downstream file, leading to
recompilation failures.
This commit fixes that by moving all that logic into the stylesheet
graph. The graph now has full and sole responsibility for providing a
consistent view of which stylesheets depend on one another even as the
shape of the graph changes, and the watcher is just a client of that
logic.
Closes#550
This allows us to support asynchronous importers and, eventually,
functions without breaking synchronous support. The copies were made
manually, but the eventual plan is to auto-generate the synchronous
versions by stripping all asynchrony from the async versions.
See #9
