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.
`find test -type f | xargs grep "Error:.*feature\/"` gives 14 cases where we report errors with paths. 9 of those cases use `/resources`, the other 5 do not. All 14 occur in  the `/Modules` tests. This makes sense because the only place where file paths are part of the es6 feature list are in modules.