resourceloader: Make action=purge affect ResourceLoaderWikiModule

Purging a wiki page included in a WikiModule should bump its version hash.
This is not needed by design, but it's a workaround until our caching layers
are more resilient against cache poisoning.

This gives users the power to resolve issues themselves.

Change-Id: I37d3379e76a6b652268c5a007e0ad741470fe6e1