Currently, Stack does not support dependency on public sublibraries. I consider it ‘No. 1’ in the list of priorities for Stack, but it has exceeded my own capabilities to date. Other people have started on the endeavor from time to time, but not yet seen it through. I think the practical problem is that ‘a package has no more than one library’ is deep-rooted in Stack’s code, and so a lot of work is required to change that: it is not a ‘bite-sized’ problem.
EDIT: To explain my prioritisation, my concern was that - as the Cabal package description format allows for public sublibraries (from cabal-version: 3.0), and Cabal (the library) supports them - then people might start producing packages that Stack cannot handle - including packages that have a central role in the Haskell ‘ecosystem’.