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.
Some tests which need path cleansing have path bits in two parts of the error message, eg
`feature/Modules/Error_ImportDefault.module.js:4:8: 'default' is not exported by 'feature/Modules/resources/a'`

The underlying question is why did we not need these to changes before.   Our error messages now include the full path, as I think that is more correct.