fdo#70393: move sane and twain to subdirs of external

Change-Id: I0876c3295416abb704ca455bdd61881ff54a0285
Reviewed-on: https://gerrit.libreoffice.org/6568
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>