Internally you call package to what is distributed as upm package and consumed with the package manager.
On the other hand, “module” is a package but embedded in the editor, that is to say, it cannot be changed or updated in that version of the editor.
Is it like that?
What I don’t understand is why you do that? It loses all the sense, if a package for being promoted to module loses the benefits of a package as to be updated at any time, we return again to the past. There are several cases like for example uitoolkit.
Think about modules being Unity editor/engine „features“. You cannot make everything a package, that would be impractical. Like for instance separate packages for the scene and the game view, the project and scene hierarchies, or the ScriptableObject, EditorWindow, Mesh and all the other APIs ….
There are also practical benefits of assuming a specific module always exists, such as not having to test for or work around cases where that module is not installed.