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.
The CodeUnit address should be the filename in the case of Node or URL in the case of browser.  I don't know if we have a test to verify this claim however.

Yes the PR strips the filename twice, because it also puts the full filename in the error message (before it was a relative path).

What error is masked? Is the related to the Windows problem?