Only load all modules when a hook gets invoked

Sort-of follow-up to #1058720: Lazy load modules which I've marked as won't fix.

Now there's the extension handler, there's the possibility to load all modules only when hooks are invoked (ideally only if there's an implementation of a hook).

This would allow module loading as well as potentially some of the system configuration data loading to be skipped for requests that either don't need to invoke any hooks at all, or invoke one or two hooks which aren't implemented.