packed modtype lookup can fail due to missing cmi

[Octachron]

Closes #9406:

This PR makes module types from missing cmis behaves the same as abstract module type everywhere.

This was already the case in nearly all cases, except for a forgotten case in the function Typemod.modtype_of_package.

After this change, there is no call site of Env.find_modtype that distinguish between the case None and exception Not_found. It would probably make sense to fold the two cases into one.

[Drup]

Sounds good. This fixes the bug and make the behavior consistent again. The patch and tests look fine.

I would prefer if missing cmis were actually an error, but that's a contentious topic for another time. :)

[gasche]

It would probably make sense to fold the two cases into one.

You are proposing that Env.find_modtype would never raise Not_found and only return None? (Are you planning to submit a PR?)

[Octachron]

Rereading the code, it would rather be a version of find_modtype_expansion that returns an option rather than raise a Not_found exception.

Maybe, we could split find_modtype into find_modtype_decl for finding the declaration of the module type and find_modtype for looking up just the type with the Not_found -> abstract translation.
I am willing to submit a PR if we agree on a design.

[Octachron]

Reviewing the fix, I would prefer to start with integrating the current bugfix PR (to integrate the test) before potentially moving on to a refactoring PR.

[gasche]

The fix looks obviously correct. Approving. This is good to go if the CI passes and the patchset is rebased appropriately, please feel free to merge. (I will probably forget to do it myself.)