Broken build when requiring moment-timezone and moment

`factor-bundle` creates broken bundles when required modules have linked dependencies. This happens for example when installing `moment-timezone` and afterwards `moment`. So `moment` exists two times in the `node_modules` folder, once for `moment-timezone` and once in `moment`. This seems to break browserify. When installing the modules the other way around the bundle does not break (this time there is only one `moment`).

I don't know if this is more of a problem with the way npm install modules when they have duplicate dependencies or if this is a problem with factor bundle.

I set up a test case over at: https://github.com/davidlanger/factor-bundle-moment-timezone

Comienza con la reproducción en https://github.com/davidlanger/factor-bundle-moment-timezone e instala moment-timezone antes que moment; después, compara el bundle resultante con el de la instalación en el orden inverso. Comprueba si la dependencia duplicada de moment rompe factor-bundle o la estructura del paquete, y considera terminado el problema cuando la reproducción produzca de forma consistente un bundle funcional.

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