Currently all of the register expression initializers depend on a single initializer, so things that need to go before expressions can depend on that. We should do the same thing for document sources.

When we register a parser using REGISTER_DOCUMENT_SOURCE, REGISTER_EXPRESSION, etc, we need to make sure the initializer declares its dependency on the "default" initializer. The "default" initializer is responsible for initializing serverGlobalParams, which we depend on for FCV and feature flags. We should make the grouped initializer responsible for this dependency.

Usually the MONGO_INITIALIZER macro handles this, but we are using MONGO_INITIALIZER_GENERAL in at least one place (to declare reverse-dependencies).