Utilities written in Python may depend on external (preinstalled) Python
packages; for example, Ecc depends on "antlr_python_runtime-3.0.1". Such
packages need not be installed system-wide, as long as they are reachable
through PYTHONPATH. Therefore we shouldn't overwrite the user's PYTHONPATH
with "BaseTools/Source/Python"; instead, we should prepend the latter to
the former.