phantomjs2: fix evaluation and refactor

The error was due to the fact that with-introduced bindings have lower
priority and we do have `darwin` in scope already.

Fixes #12350. Closes #12351. (A slightly different fix.
I chose this to lower the risk of people re-introducing the mistake.)

pkgs/development/tools/phantomjs2/default.nix

···
     $out/bin/phantomjs
   '';
 
-  meta = {
+  meta = with stdenv.lib; {
     description = "Headless WebKit with JavaScript API";
     longDescription = ''
       PhantomJS2 is a headless WebKit with JavaScript API.
···
     '';
 
     homepage = http://phantomjs.org/;
-    license = stdenv.lib.licenses.bsd3;
+    license = licenses.bsd3;
 
-    maintainers = [ stdenv.lib.maintainers.aflatter ];
-    platforms = with stdenv.lib.platforms; darwin ++ linux;
+    maintainers = [ maintainers.aflatter ];
+    platforms = platforms.darwin ++ platforms.linux;
   };
 }