Set __file__ when source'ing a Python script

This patch arranges to set __file__ when source'ing a Python script.
This fixes a problem that was introduced by the "source" rewrite, and
then pointed out by Lancelot Six.

Reviewed-by: Lancelot Six <lancelot.six@amd.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>