Extract flake dependencies from the 'outputs' arguments

That is, instead of

  inputs = [ "nixpkgs" ];

  outputs = inputs: ... inputs.nixpkgs ...;

you can write

  outputs = { nixpkgs }: ... inputs.nixpkgs ...;

src/libexpr/eval.hh

@@ -80,7 +80,7 @@ public:
         sFile, sLine, sColumn, sFunctor, sToString,
         sRight, sWrong, sStructuredAttrs, sBuilder, sArgs,
         sOutputHash, sOutputHashAlgo, sOutputHashMode,
-        sDescription;
+        sDescription, sSelf;
     Symbol sDerivationNix;
 
     /* If set, force copying files to the Nix store even if they