haskellPackages.ghcWithHoogle: add `passthru.hoogle`

Package sets built with `haskellPackages.ghcWithHoogle` provide a Hoogle
executable which points to a Hoogle database, but there's no way to
programmatically access the derivation containing the Hoogle database
from the package set.

This adds a `passthru.hoogle` attribute to package sets built with
`haskellPackages.ghcWithHoogle`, and a `passthru.database` helper to
derivations built with `haskellPackages.hoogleWithPackages`.

pkgs/development/haskell-modules/hoogle-local-wrapper.sh

···
 
 COMMAND=$1
 shift
-exec @hoogle@/bin/hoogle "$COMMAND" --database @out@/share/doc/hoogle/default.hoo "$@"
+exec @hoogle@/bin/hoogle "$COMMAND" --database @database@ "$@"

pkgs/development/haskell-modules/hoogle.nix

···
   buildPackages,
   haskellPackages,
   writeText,
+  runCommand,
 }:
 
 # This argument is a function which selects a list of Haskell packages from any
···
       # we grab the doc outputs
       (map (lib.getOutput "doc") packages);
 
+  # Hoogle database path, relative to `$out`.
+  databasePath = "share/doc/hoogle/default.hoo";
+
 in
-buildPackages.stdenv.mkDerivation {
+buildPackages.stdenv.mkDerivation (finalAttrs: {
   name = "hoogle-with-packages";
   buildInputs = [
     ghc
···
       )
     }
 
+    databasePath="$out/"${lib.escapeShellArg databasePath}
+
     echo building hoogle database
-    hoogle generate --database $out/share/doc/hoogle/default.hoo --local=$out/share/doc/hoogle
+    hoogle generate --database "$databasePath" --local=$out/share/doc/hoogle
 
     echo building haddock index
     # adapted from GHC's gen_contents_index
···
     echo finishing up
     mkdir -p $out/bin
     substitute ${wrapper} $out/bin/hoogle \
-        --subst-var out --subst-var-by shell ${stdenv.shell} \
+        --subst-var-by shell ${stdenv.shell} \
+        --subst-var-by database "$databasePath" \
         --subst-var-by hoogle ${hoogle}
     chmod +x $out/bin/hoogle
   '';
 
   passthru = {
     isHaskellLibrary = false; # for the filter in ./with-packages-wrapper.nix
+
+    # The path to the Hoogle database.
+    database = "${finalAttrs.finalPackage}/${databasePath}";
+
+    tests.can-search-database = runCommand "can-search-database" { } ''
+      # This succeeds even if no results are found, but `Prelude.map` should
+      # always be available.
+      ${finalAttrs.finalPackage}/bin/hoogle search Prelude.map > $out
+    '';
   };
 
   meta = {
···
     hydraPlatforms = with lib.platforms; none;
     maintainers = with lib.maintainers; [ ttuegel ];
   };
-}
+})

pkgs/development/haskell-modules/with-packages-wrapper.nix

···
 let
   inherit (haskellPackages) llvmPackages ghc;
 
-  packages =
-    selectPackages haskellPackages
-    ++ lib.optional withHoogle (hoogleWithPackages selectPackages);
+  hoogleWithPackages' = if withHoogle then hoogleWithPackages selectPackages else null;
+
+  packages = selectPackages haskellPackages ++ [ hoogleWithPackages' ];
 
   isGhcjs = ghc.isGhcjs or false;
   isHaLVM = ghc.isHaLVM or false;
···
     preferLocalBuild = true;
     passthru = {
       inherit (ghc) version meta;
+
+      hoogle = hoogleWithPackages';
 
       # Inform users about backwards incompatibilities with <= 21.05
       override =