GHC loads packages lazily, so at the end of compilation (at least in --make mode) it knows all the dependencies, required to compile the code. See eps_PIT in ExternalPackageState. If we compare this list with the list of packages, passed on command like via --package-id, then we'll get a list of unused dependencies.

I propose to add a flag -Wunused-packages, off by default, which will report the unused packages if any.