Fix how we locate and link node.js

So I got a bit of an ear-full yesterday about us pulling dependancies from source repos during our build. We need to come up with a cleaner, more proper (tm) way to link with our dependancies.

For node.js, we're going to have a lot of resistance from security (and probably other fronts) doing what we're doing now (embedding it into our own launcher). I think we need to shift our focus to a pure node module implementation executed via a system installed node instance. Of course then node will need to be packaged into the ubuntu archive, but I think thats the only direction people are willing to accept anyway. For now, lets work with the snapshot we have in lp:unity-js-scopes/node.js (v4.0.0). I think this is the version we should push to get archived.

Related (but not duplicate) bug: Bug #1482158