This repository demonstrates a possible bug in ghc-pkg, by showing 4 possible linking usecases, out of which the library-archive-linking usecase requires a non-ideal method to work.
This is described in much more detail on my blog.
Notice the package.yaml file in each of the 4 directories. It reveals important information about which component is linked to the library. <dir-name>.cabal mostly mirrors the package.yaml file (it is generated automatically by hpack) so you can ignore that. Please remember to make the appropriate type of libraries (archive/shared) in each of the folders according to the README files.