I've changed my mind with time. I think it's better to let the template (100% customized by user) to handle most the inclusions needed by widgets, including external libraries and such. The auto-including logic for printWidgetSource templates sometimes becomes a problem because the code is never compressed.
We could have something to handle including widget code for 'dev' purpose, but for the rest it should be controlled by the user.
I'm moving this to 'Future' until we think that is becoming an issue at some point.