Also combine the two coffee registerCompiler calls into one
it's a bad on-disk cache that should be improved to not write out all
the unchanged files every time too. but the tests do show that it skips
the unnecessary recompile
less doesn't use an on-disk cache yet
Specifically, ensure that we don't even include client-specific files in
the server WatchSet by making sure we pass a precise set of extensions
to initFromAppDir. We still also rule out wrong-arch files at a later
stage too (because sourceExtensions only affects initFromAppDir).
Without the change to compiler.js, the final test (adding
yet-another-root.main.less) restarts the server rather than printing
"Client modified -- refreshing".
