watcher rewrite: Add WatchSet and readDirectory.
rewrite Watcher
twiddle exports
get watch-test to pass
remove dead builder.copyDirectory({depend:true}) code
Checkpoint for actually using WatchSets.

Have not yet touched initFromAppDir.
checkpoint for initFromAppDir rewrite
in theory, this might work
fix minor bugs. bundler-test passes!
watch nonexistent dirs better
fix HTML sort hack. run-tools-tests passes!
Don't put the old pluginWatchSet onto a Package until we know we're using the
unipackage.