hpack: add a statically built executable

hpack is a little tool to convert a yaml spec to a cabal file, but its
runtime closure depends on ghc if it’s not built statically.

pkgs/top-level/all-packages.nix

···
 
   hdl-dump = callPackage ../tools/misc/hdl-dump { };
 
+  hpack = haskell.lib.justStaticExecutables haskellPackages.hpack;
+
   hpcg = callPackage ../tools/misc/hpcg/default.nix { };
 
   hpl = callPackage ../tools/misc/hpl { };