Add check that a module doesn't try to override itself.

If that happens it will be left out entirely.

It would be nice to catch this on the Soong level, but there are
several separate code paths there so this ensures it isn't missed.

Test: `m nothing` with packages/modules/ArtPrebuilt/Android.bp that
  had this mistake
Bug: 172480615
Change-Id: I2d31679b2ca5b57bf14f35703c10093275209890