Fix js/non-js import problem

[bkardell]

The hitch server respects requests for the hitch whether they include .js or not at the end.

The runtime engine also guards you from downloading two copies of the same hitch...

That's fine, but currently you can use both full URIs or shorted package: URIs and they don't treat things the same, so the guard doesn't work.

It may also be worth considering what this means to versioning... Is it even conceivable that two versions of the same hitch could be mixed in the same page? I kind of think not.