Isolates the file loader so it does no longer depend on the parser.
This frees the loader to only care aout loading files, which makes it
easier to inject custom loaders via the provided interface.

Since the loader is now only a single function that is extracted
immediately, it's been reduced to a function instead of a class.

The previous used version of babel had a bug with transpiling async
methods on classes. See:

babel/babel#6806

Update of babel dependencies fixes the problem

Since the parser shouldn't have to deal with file system operations, the
loader plugin now has to provide a unique id for an import, which can
just be an absolute path for the default implementation.