Warn about home module not listed on command line
It is an attempt to fix this cabal issue: https://github.com/haskell/cabal/issues/1746
Basically, when user fails to list all modules in exposed-modules or other-modules, cabal produces broken package.
The idea of the fix is to teach GHC to warn about home modules (not from an other package), that are not explicitly listed in command line.
Option name -Wmissing-home-modules is a subject for bikeshedding.