`ghc-pkg list` tells us everything hackage2nix needs to know. In the
past the core-packages list and compiler setting in hackage2nix was
maintained manually which inevitably leads to it being forgot once in a
while – this will then mess with flag resolution when generating the
package set in some cases. Luckily, we can just let a simple derivation
do this for us.
Resolves#202621.
b7e41b825e