title: What's the use of _moduleIndices2 and _moduleIndices in webpack
body: <p>i read Webpack source code about buildChunkGraph,i find chunkgroup have two attr called _moduleIndices2  and _moduleIndices ,but i only found them be set in buildChunkGraph function,
  no place to use it,i am confused about _moduleIndices2 and _moduleIndices attr.</p>
  <p>Thank anyone in advance for helping me.</p>