refactor(select): rename source files to follow convention

All `[ng-model]` directive source files under `lib/directive` have a
name of the form `ng_model*.dart`. Rename:
- `input_select.dart` to `ng_model_select.dart`.
- `input_select_spec.dart` to `ng_model_select_spec.dart`.

Closes #871

Author: chalin

lib/directive/module.dart

@@ -39,7 +39,7 @@ part 'ng_src_boolean.dart';
 part 'ng_style.dart';
 part 'ng_switch.dart';
 part 'ng_non_bindable.dart';
-part 'input_select.dart';
+part 'ng_model_select.dart';
 part 'ng_form.dart';
 part 'ng_model_validators.dart';