Nix integration cleanup

Keep the shell used by Stack defined in one place.

package.nix

@@ -40,6 +40,6 @@ in
   app = hlib.justStaticExecutables hpkgs.stackage-server;
   shell = hpkgs.shellFor {
     packages = p: [ p.stackage-server ];
-    buildInputs = [ pkgs.cabal-install pkgs.haskell-language-server pkgs.ghcid pkgs.haskellPackages.yesod-bin ];
+    buildInputs = [ pkgs.cabal-install pkgs.haskell-language-server pkgs.ghcid pkgs.haskellPackages.yesod-bin pkgs.postgresql ];
   };
 }

shell.nix

@@ -0,0 +1,7 @@
+(import (
+  fetchGit {
+    url = "https://github.com/edolstra/flake-compat";
+  }
+) {
+  src =  ./.;
+}).shellNix

stack.yaml

@@ -30,8 +30,4 @@ allow-newer-deps:
   - pantry
 
 nix:
-  packages:
-    - zlib
-    - postgresql
-    - pkg-config
-    - cacert
+  shell-file: shell.nix