fdo#70393: move rhino to a subdir of external

Change-Id: I7d7858d84a62568d9b0c61403645da13658e78d2
Reviewed-on: https://gerrit.libreoffice.org/6552
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>