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.cc

@@ -304,6 +304,7 @@ EvalState::EvalState(const Strings & _searchPath, ref<Store> store)
     , sOutputHashAlgo(symbols.create("outputHashAlgo"))
     , sOutputHashMode(symbols.create("outputHashMode"))
     , sDescription(symbols.create("description"))
+    , sSelf(symbols.create("self"))
     , repair(NoRepair)
     , store(store)
     , baseEnv(allocEnv(128))