FIX #46: Fallback to index.js if no main attribute in package.json

I see, so you want to try foo.js when you have main: foo in package.json.

Could you please rewrite this to conform to the pattern used elsewhere in this file, which would be something like:

seq
  (fun () -> path_if_exists path)
  (fun () -> seqf
      (fun ext -> path_if_exists (path ^ ext)) (* try all extensions *)
      Files_js.flow_extensions)