This is due to the way widgets are bundled through Dojo build. There is no way you can include an external library like that, dojo build will fail. This same issue we have had in the past and there is no simple way around it. If you want to include Google Maps, you should include it locally.
Luckily we have a way around this. Have a look at our Google Maps Widget. This includes a local copy of the JSAPI, which will load the Maps library (or any other that you like), for you.
JSAPI is included as a required dependency
This will create a global (window) variable google