Terminology for third-party modules is confusing

[matrixbot]

This issue has been migrated from #12186.

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Currently we have a module system (or interface) where modules can register callbacks and use module API methods, which are different things. It feels like we keep confusing them, for example calling a module callback a module API (which is even more confusing because it technically is an API but just isn't a method of the ModuleApi class).

We need to figure out a better terminology.