aspellWithDicts: create derivation with aspell and selected dictionaries

Currently, `aspell` checks the active profiles for dictionaries. While
this may be convenient, it does not work with `nix-shell` and it doesn't
allow any isolation.

This commit adds the possibility to use composition by creating a
derivation with `symlinkJoin` that contains all the chosen dictionaries,
and another derivation that wraps the executables linking to the dictionaries.

Nix example:

my_aspell = aspellWithDicts(ps: with ps; [ en nl ])

`nix-shell` example:

nix-shell -p 'aspellWithDicts(ps: with ps; [ en nl ])'

pkgs/development/libraries/aspell/aspell-with-dicts.nix

@@ -0,0 +1,35 @@
+# Create a derivation that contains aspell and selected dictionaries.
+# Composition is done using `pkgs.buildEnv`.
+
+{ aspell
+, aspellDicts
+, makeWrapper
+, symlinkJoin
+, runCommand
+}:
+
+f:
+
+let
+  # Dictionaries we want
+  dicts = f aspellDicts;
+
+  # A tree containing the dictionaries
+  dictEnv = symlinkJoin {
+    name = "aspell-dicts";
+    paths = dicts;
+  };
+
+in runCommand "aspell-env" {
+  buildInputs = [ makeWrapper ];
+} ''
+  # Construct wrappers in /bin
+  mkdir -p $out/bin
+  pushd "${aspell}/bin"
+  for prg in *; do
+    if [ -f "$prg" ]; then
+      makeWrapper "${aspell}/bin/$prg" "$out/bin/$prg" --set ASPELL_CONF "data-dir ${dictEnv}/lib/aspell"
+    fi
+  done
+  popd
+''

pkgs/top-level/all-packages.nix

@@ -7393,6 +7393,8 @@ with pkgs;
 
   aspellDicts = recurseIntoAttrs (callPackages ../development/libraries/aspell/dictionaries.nix {});
 
+  aspellWithDicts = callPackage ../development/libraries/aspell/aspell-with-dicts.nix { };
+
   attica = callPackage ../development/libraries/attica { };
 
   attr = callPackage ../development/libraries/attr { };