Bundle external dependencies

In my project I'm using both **npm** modules and **bower** components (with debowerify) which I want to bundle together as `vendor.js` while keeping my own code in an `app.js` bundle.

What I did so far is created the `app.js` bundle by setting `external` to the **npm** modules and **bower** components. To figure out what modules to set as `external` I read the dependencies from `package.json` and `bower.json`.

Now, in a similar fashion I would like to parse my codebase, see which requires are external modules and bundle them together.
How could I do this?