fix: %{cmi:...} and other module artifact pforms under (include_subdirs qualified)

[Alizter]

• Fixes module artifact targets %{cmi:...} etc. crash on duplicate leaf module names #14496
• Depends on test: building module artifact targets under (include_subdirs qualified) #14497

Under (include_subdirs qualified), building any module artifact pform target (%{cmi:...}, %{cmo:...}, %{cmx:...}, %{cmt:...}, %{cmti:...}) crashes with Map.add_exn: key already exists when two qualified subdirectories define modules with the same name.

Also reproduces in this repo: dune build %{cmi:bin/main} now works (bin/ uses (include_subdirs qualified) and contains four group.ml files in different qualified subdirs).

[shonfeder]

I have one suggestion to improve (imo) readability, which is optional, but requests about test coverage for user errors, which, given my current understanding, should be addressed.

[Alizter]

I need to work out two things to make progress on this PR:

1. Get rid of the ad-hoc path/module-path conversion. We do this properly for include_subdirs qualified somehwere.
2. Consider accepting a wider range of syntax. We currently allow %{cmo:foo/bar} in line with include_subdirs unqualified, however for qualified we may want to allow some sort of module path here too, such as %{cmo:foo.bar}. It will need to be thought about.

[Alizter]

Okay I've addressed (1), but (2) is trickier. I thought about ways we can mix paths and module paths like %{cmi:foo/bar.zar} but it got tricky. I think just having %{cmi:foo/bar/zar} is less confusing and is already what we do for unqualified.

[shonfeder]

This looks like a really nice simplification over the first iteration! The only change that I think of as blocking here as the release note. Other suggestions are just stylistic or subjective readability notes.

I would like clarification my question about the safety of add_exn tho!

[shonfeder]

LGTM. Thanks for the explanations!