Private modules & External Virtual libraries do not work

[rgrinberg]

Implementing an external virtual library that contains private libraries currently does not work. The reason for this is how we install private modules. We install the .cm[ox] in the main install directory of the library, but when we restore the Lib_modules.t for the library, our code assumes the private objects will exist in the .private dir.

@bobot Your change to store all the .cm[ox] files in 1 directory should fix this I believe. How is that coming along? Is it possible to quickly separate that work from the rest of the interfaces stuff?

[bobot]

It is already separated, it is #1676 (just all the cmx are in one directory and all the cmo in another). I need to check the installation part, and fix the current error found by the CI. You can take a first look. If we don't want to keep the simple layout when there is no private module, it is feature complete. But otherwise more refactoring is needed (obj_dir in module.ml perhaps)

[rgrinberg]

This is now fixed.