lib/options: Better mergeEqualOption error for uncomparable types

For an option definition that uses `lib.options.mergeEqualOption`
underneath, like `types.anything`, an error is thrown when multiple
functions are assigned, indicating that functions can't be compared for
equivalence:

  error: The option `test' has conflicting definition values:
  - In `xxx': <function>
  - In `xxx': <function>
  (use '--show-trace' to show detailed location information)

However, the error message didn't use the correct files. While above
error indicates that both definitions are in the xxx file, that's in
fact not true. Above error was generated with

  options.test = lib.mkOption {
    type = lib.types.anything;
  };

  imports = [
    {
      _file = "yyy";
      test = y: y;
    }
    {
      _file = "xxx";
      test = x: x;
    }
  ];

With this change, the error uses the correct file locations:

  error: The option `test' has conflicting definition values:
  - In `xxx': <function>
  - In `yyy': <function>
  (use '--show-trace' to show detailed location information)

lib/options.nix

@@ -11,6 +11,7 @@ let
     filter
     foldl'
     head
+    tail
     isAttrs
     isBool
     isDerivation
@@ -144,7 +145,7 @@ rec {
       if def.value != first.value then
         throw "The option `${showOption loc}' has conflicting definition values:${showDefs [ first def ]}"
       else
-        first) (head defs) defs).value;
+        first) (head defs) (tail defs)).value;
 
   /* Extracts values of all "value" keys of the given list.