[IMP] account_edi_{facturx,ubl}: backport usage of _retrieve_* methods

a9a46cf0 introduced methods _retrieve_{partner,product,tax} but they
were not used in 14.0, whereas they are used from saas-14.1.

This commit changes the account_edi_facturx and _ubl modules so that
they make use of these methods.

Part-of: odoo/odoo#75738