Allow consumers to supply a haskell package override
Recently I had to set allowBroken = true
in tahoe-great-black-swamp because the pinned nixpkgs has a Haskell package it depends on marked as broken even though it is not. It would have been better if I could have passed in a Haskell package override that unset the broken flag for that single package. More generally, consumers might want to adjust the version of something or add a whole new package (apart from the one they are defining) to the package set. hs-flake-utils should support this.