fdo#70393: move external accessories to a subdir of external

Change-Id: If69c8e10eb3ce19208390f63aef49cf1d9554ef1
Reviewed-on: https://gerrit.libreoffice.org/6572
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>