chore: generalize node6 transpilation (#1560)

This patch unifies node6 transpilation:
- instead of generating multiple top-level directories, prefixed with
  `node6-`, all transpiled code gets placed under single `node6/` folder
- transpilation doesn't change require paths of transpiled modules any
  more

lib/Downloader.js

@@ -34,7 +34,8 @@ const downloadURLs = {
   win64: '%s/chromium-browser-snapshots/Win_x64/%d/chrome-win32.zip',
 };
 
-const PROJECT_ROOT = path.join(__dirname, '..');
+// Project root will be different for node6-transpiled code.
+const PROJECT_ROOT = fs.existsSync(path.join(__dirname, '..', 'package.json')) ? path.join(__dirname, '..') : path.join(__dirname, '..', '..');
 
 class Downloader {
   /**