Keep WfdMetaResource instances in the regular metadata file

[mpdude]

#45 broke the always_expire_wfd_meta_resources feature in a subtle way:

The change made the WfdMetaConfigCacheFactory decorate the regular config_cache_factory service unconditionally. The idea was that the CacheBustingResourceChecker would then be added when setting always_expire_wfd_meta_resources: true, causing caches that rely on wfd_meta to be refreshed every time.

What we missed was that WfdMetaConfigCache sorts the resources into two groups - those for the regular freshness check (which happens by means of the Symfony ConfigCacheFactory), and the "extra" resources checked with wfd_meta data.

WfdMetaResources would always end up in the second group, which is checked by WfdMetaConfigCache, and this check was not aware of ResourceCheckers at all. This rendered the CacheBustingResourceChecker useless, since it was never applied to anything.

This PR fixes the problem by keeping WfdMetaResource instances in the set of resources to be checked by the "inner" (regular) config cache factory, where they can be picked up by CacheBustingResourceChecker.

It also gives a bit more clarity to how the WfdMetaConfigCache actually works: It now behaves more cleanly like a decorator around the "inner" ConfigCache instance, delegating the isFresh() check to that implementation first and then running a second round of checks.