Based on the conversation at T384184#10522794 et seq, CommunityConfiguration should allow two ways to access configuration:
- MediaWikiConfigReader, which would be used to access only CC-based configuration; for non-CC configuration, it should throw a ConfigException
- MediaWikiConfigRouter, which would determine whether a configuration is CC based or not, and either route it to MediaWikiConfigReader or to the MainConfig service
The first would be used in cases when a consumer is certain only CC config is needed. The other one would be used to make it easier to switch between MainConfig and CC (to ensure there is no need to audit all usages; one merely needs to add the config to CC).