With the following project:

./src:
dune       # containing (include_subdirs qualified)
x.ml

./src/a:
y.ml

./src/a/b:
z.ml

x.ml would see y.ml as A.Y and z.ml as A.B.Z. Then y.ml would see z.ml as B.Z but couldn't see x.ml.

This should allow to organize the modules using the FS layout.

I'm interested in this as it would allow to port Camlp4 to dune, which would simplify it's maintenance, and it's also a nice feature to have. However, I'm not planning to work on this in the near future, so anyone should feel free to assign themselves this feature if they are interested in it.