Use scoped sourceDirectories in purescript task

I think this is the right thing to do. It makes it easier to include other source directories only for purescript files, and shouldn't break anything or cause any incompatibilites.

I ended up just putting a simple bower run in resourceGenerators and wanting to add all the bower dependencies during purescript compilation, without picking up js files from there.  There may be a better way to do all this -- I haven't used bower before.

I am somewhat familiar with js-engine and sbt-web, though, and noticed some of your other issues related to them, so if I make more progress with purescript (just playing around for now), I may be able to help.