I added getPackageDir to utils on tickets/
DM-2635 and included a simple unit test.
I chose to implement the function using environment variables, rather than using the productDir method that sconsUtils adds to eups. If standard eups included productDir as a method, or the function was clearly part of sconsUtils instead of having sconsUtils add it to eups, I would have chosen differently, because it makes me nervous to have two different implementations. But at least the implementation is trivial and the sconsUtils version will only be used inside of sconsUtils once
DM-2636 is implemented.