New class ScribuntoHooks implements ScribuntoExternalLibrariesHook
A separate class avoids a hard dependency on extension Scribunto.
This change requires a phan dependency on extension Scribunto in
project integration/config in file zuul/parameter_functions.py:
I051ca19c8988d6004f991485181e48f877b36b60
Change-Id: Iff19d6b0d3aab82c232feb615d19696183d9d2a9