...on indirect dependencies too.

Here a self reference to any formula-group
means if there are any references in a formula
(of the formula-group itself or any of its dependencies)
that points to any element inside the formula-group.
If there are any self-references, then that formula-group
can't be computed in parallel.

For example, with this patch we can detect the following case:-
  Suppose the formula-group that we want to check is:
  "=(F2+G2-10)*10.0" spanning A2:A100. Let the formula-group
  starting at F2 be "=A1*0.1-10". The indirect dependency
  formula-group starting at F2, references back the elements of
  our original formula-group at A2. This makes the F.G at
  A2 unsafe for parallel computation.

Concretly, this patch fixes a recalc crash on tdf#63638/1

Change-Id: I7b999a34571b191d2f70da6a3831f78b24a6b0a7
Reviewed-on: https://gerrit.libreoffice.org/54433Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Jenkins <ci@libreoffice.org>