While implementing ArraynessAPI I saw that there is at least one helper to `MultikeyPaths` that is implemented outside `src/mongo/db/index/multikey_paths.cpp` and namespace multikey_paths.

Specfically here could be safely moved to multikey_paths.cpp/.h

Check for any other functions acting on top of MultikeyPaths and move them to a single library, and adjust their invocations accordingly.