Allow for dynamically-loaded PHP libraries

Scribunto currently supports libraries with PHP callbacks that are
loaded on startup, and pure-Lua libraries that may be loaded from the
module with require().

This change allows for libraries with PHP callbacks to also be loaded
with require().

Change-Id: Ibdc1f4ef51b1c8644c3d4c98d57755b5c06447a5
Merge "Allow for dynamically-loaded PHP libraries"