[FIX] l10n_ar: return vat info grouped by tax

In the method _get_vat(), used to inform AFIP about the vat taxes of an
invoice, we were assuming that in the journal items of the invoices
there is always one line per tax group, but if the user enables the
option "Include in Analytic Cost" of the tax, then it could create more
than one line per tax so it will return an error if you try to validate
the invoice in AFIP or upload the vat book.

closes odoo/odoo#75802

Signed-off-by: Josse Colpaert <jco@openerp.com>