Allow '.js' suffix in packages names

Almost nothing is actually done but I don't want to proceed until there is certainty that such changes can be done in RequireJS.

From my point of view, there are several reasons to allow ".js" suffix:
* It will make possible to publish and install packages with ".js" suffix via [Jam](http://jamjs.org/) (rels https://github.com/caolan/jam/issues/97)
* It will bring more consistency with Node.js, packages of which may end in ".js"
* It can hardly break any existing configurations