fdo#70393: move xsltml to a subdir of external

Change-Id: I2d244cc2ea9390f3a118bb620d397faa59e4e491
Reviewed-on: https://gerrit.libreoffice.org/6555
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>