[Packager] Refactor packager (separate preprocessor step)

[jtremback]

Maybe I'm messing something up, but I have not been able to get the packager to work with es6 modules, or anything requiring the Babel runtime. From digging around in the packager a little more, I believe this has to do with the fact that it runs the transform on each file individually.

I am able to this stuff working by running Babel on my files myself before they reach the packager, but it seems messy to preprocess it twice.

[jtremback]

I need to investigate this further, will submit a PR if I pursue it.