scons: validate buildscripts/requirements.txt

Python's "pip" system is difficult to work with.

It can be hard to tell if the dependencies specified by buildscripts/requirements.txt are actually met by the python environment scons will use, and scons can verify this on its own.

Maybe via the pkg_resources API https://stackoverflow.com/questions/16294819/how-to-check-if-my-python-has-all-required-packages