haskell-generic-builder: add a passthru attribute 'isHaskellLibrary' to every derivation

The presence of this attribute allows us to recognize Haskell derivations.
Furthermore, we can use it to distinguish libraries from executables (which is
useful for the code that's generating the wrapper).

pkgs/development/haskell-modules/generic-builder.nix

@@ -250,6 +250,8 @@ stdenv.mkDerivation ({
 
     inherit pname version;
 
+    isHaskellLibrary = hasActiveLibrary;
+
     env = stdenv.mkDerivation {
       name = "interactive-${optionalString hasActiveLibrary "haskell-"}${pname}-${version}-environment";
       nativeBuildInputs = [ ghcEnv systemBuildInputs ];