fdo#70393: move vigra to a subdir of external

Change-Id: I98e2ae25865c4ec38da19a24ba4347540d170e4f
Reviewed-on: https://gerrit.libreoffice.org/6554
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>