Don't allow flake inputs to have both a flakeref and a follows

Having both doesn't make sense so it's best to disallow it. If this
causes issues we could turn into a warning.

src/libflake/flake.cc

@@ -154,6 +154,9 @@ static FlakeInput parseFlakeInput(
             input.ref = parseFlakeRef(state.fetchSettings, *url, {}, true, input.isFlake, true);
     }
 
+    if (input.ref && input.follows)
+        throw Error("flake input has both a flake reference and a follows attribute, at %s", state.positions[pos]);
+
     return input;
 }