fdo#70393: move languagetool to a subdir of external

Change-Id: I402682f21d16c3069d5b6885bd75998bd2e16b3f
Reviewed-on: https://gerrit.libreoffice.org/6544
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>