fix(bundler): don't trace dependencies which have no main file

[JeroenVinke]

closes #435 (comment)

I need to do some final tests and rebase it on master

[JeroenVinke]

#435 (comment) is a better solution. I'll update this PR

[zewa666]

Wow, that fix, so simple yet so effective.

Fresh TS scaffold with:

{
  "name": "rxjs",
  "path": "../node_modules/rxjs",
  "main": "Rx"
}

648kb vendor file

and with main: false 410kb.

of course its just a demo using only minimal Rx dependencies, but I still expect at least 100-150kb savings on a typical RxJS project.

You better get that merged asap :)