Hm. Perhaps I need to go through the control file and be a lot more careful about the versioning to make it match (can I set versions on the individual subpackages built?). While I wasn't the first one to throw these all into the same package, after trying to package these separately futilely myself, I am guessing the reasoning is that these packages all have extraordinarily tight build dependencies on each other, and all build more or less exactly the same way. As a result, one of the easiest ways to build is to pull all the different pieces in. Since users primarily install binaries, it doesn't really pose a bigger burden on them, though I suppose it might add to the burden of the buildbots.
I saw notes in the TODO for the debian-ocaml-maint janest-core repo listing exporting more libs as an item, so I assumed that it was acceptable to migrate other librares that had a source dir in this package into being provided by the package.
I would like to hear from some of the debian ocaml maintainers before I attempt to split this though, as they've been at this longer than I have and may be able to more cleanly articulate how the multi-source package was intended to work.