nix edit: Support non-derivation attributes

E.g.

  $ nix edit .#nixosConfigurations.bla

now works.

(cherry picked from commit d2032edb2f)

src/nix/command.hh

@@ -3,17 +3,12 @@
 #include "args.hh"
 #include "common-eval-args.hh"
 #include "path.hh"
+#include "eval.hh"
 
 namespace nix {
 
 extern std::string programPath;
 
-struct Value;
-class Bindings;
-class EvalState;
-struct Pos;
-class Store;
-
 /* A command that requires a Nix store. */
 struct StoreCommand : virtual Command
 {
@@ -48,7 +43,7 @@ struct Installable
 
     Buildable toBuildable();
 
-    virtual Value * toValue(EvalState & state)
+    virtual std::pair<Value *, Pos> toValue(EvalState & state)
     {
         throw Error("argument '%s' cannot be evaluated", what());
     }