Use -m flag to run script in Python module

See https://ncoghlan_devs-python-notes.readthedocs.org/en/latest/python_concepts/import_traps.html for an explanation.  Calling a Python script inside a
package directly causes modules to be imported using two different names,
which then causes problems with comparisons (such as isinstance()) or with
state management.
Add --validate flag to balc executable