Cache key not dependent on callback args and vice versa

Having the cache key at least internally not factor in the `$callback_args` means this could behave unexpectedly when different callback args are passed, but the same cache key is provided.

I think this should be avoided as it's too easy to make a mistake and create difficult to debug problems.

The issue does not name any files, tests, or entry points. Start by locating the cache API that accepts the cache key and callback arguments, then determine how those values are currently related. Done should prevent identical cache keys with different callback arguments, and vice versa, without introducing unexpected cache behavior.

Written by the indexing model from the issue text.