[Autofac] Allow service locator injection through the new AddOnProvidedServiceLocator, which scans the GAC for assemblies matching the *.ServiceLocator.DLL pattern.
- When many ServiceLocator DLLs are found in GAC, we use a special key stored inthe SPSite's root web property back to determine this SPSite-context's preferred service locator assembly
- To provide a real service locator to a AddOnProvidedServiceLocator, your AddOn.ServiceLocator.DLL assembly must have an implementation of the ISharePointServiceLocatorAccessor.
- Still need to test these concepts properly