DirMod.run_init() and DirModGui.run_init_gui() use exec(code) to run
addon Init.py and InitGui.py files, but don't pass __file__ in the
globals dict. Addons that reference __file__ (e.g. Silo) crash with
NameError: name '__file__' is not defined.
Pass {"__file__": str(init_py)} as the globals argument so __file__
is available in the executed code, matching the behavior of normal
Python module loading.
