code: public VirtualFile getFile(VirtualFile mountPoint, VirtualFile target) {
          final String path = target.getPathNameRelativeTo(mountPoint);
          return rootNode.getFile(new Path(path), mountPoint);
      }
docs: Get the VirtualFile from the assembly.  This will traverse VirtualFiles in assembly
  to find children if needed.
  
  @param mountPoint
  @param target
  @return
  @throws IOException