Reorg TraceurLoader along the lines outline in es-discuss.

Loader: interface with spec functions .import(), .module().
Loader impl is InternalLoader. InternalLoader focused on dependencies.
TraceurLoader extends Loader implements normalize/locate/fetch.
InternalLoader uses Loader interface for .normalize() et al.
InternalLoader uses new LoaderCompiler,  internal API for Loader to Compiler.
(Renamed LoaderHooks -> LoaderCompiler, no longer related to es6 'hooks').
Remove obsolete InterceptOutputLoaderCompiler and repl.js.
Nothing, but it does it asynchronously ;-)

The caller should expects a promise and tests the result before proceeding in the load. We don't implement the caller correctly but I was hoping we might be able to swap in es6-module-loader `Loader` some day, at least outside of bootstrapping.