Support sublibraries as dependencies

[coot]

I see failures to build io-sim with the following error on the hackage server:

Configuring io-sim-1.9.1.0...
Error: cabal: Encountered missing or private dependencies:
io-classes:{io-classes, si-timers, strict-stm} ==1.9.0.0

Full build report is available here.

Locally io-sim builds fine with cabal (after a clean cabal get io-sim-1.9.1.0). It also builds with GitHub actions, ref). I use cabal-3.14.2.0 and the same version is used by the GitHub actions.