#13669: use compilation unit name in dependency check for the native linker

[Octachron]

When checking dependency at link time, the native linker is currently using the full path for computing provided compilation unit in presence of packed modules. This creates a bug when compiling fragment of packed module separatedly

ocamlopt -for-pack Pack a.ml -c -o a.cmx
ocamlopt -for-pack Pack main.ml -c -o main.cmx
ocamlopt a.cmx main.cmx -o main

because the a.cmx file recorded that it was providing Pack.A whereas Main was registering a dependency on A directly. Consequently, the linking check for dependency emits a mysterious error

Error: No implementation provided for the following modules:
         A referenced from Main (main.cmx)

in the scenario above. This scenario happens in coccinelle build system and me and @lthls agreed that it was better to not break coccinelle even if don't really support this use case.

In order to fix this issue, this PR homogenize the notion of provided and required units for the native linker by stating to the dependency checker that a.cmx provides the compilation unit A rather than the pack submodule Pack.A . Indeed, pack submodule cannot appear as dependency in other modules: pack siblings depends will depend directly on the non-qualified path, whereas external modules will depend only on the pack module itself.

Another option would be to update the dependency side, but the changes felt too intrusive to me for 5.3 .

[lthls]

I think a comment in the test that we only support that for backwards compatibility reasons would be nice, apart from that everything looks fine.