FIX: invoking a widget with data-part overrides within a data-part

invoking a widget with data-part overrides within a data-part does not work.

example:
<div data-bind="widget: outer">
 <div data-part="outer-widget-part">
  <div data-bind="widget: inner">
   <div data-part="inner-widget-part">
     this will not take effect
   </div>
  </div>
 </div>
</div>

reason: the outer widget consumes the data-part of the inner widget.

solution: ignore data-parts within widget bindings