[FIX] base: do not generate auto xmlid for custom fields

For custom fields, the _module attribute is False, this means that
during the reflection of the model, the custom field's xmlid would be
set up as being introduced in the module of its model's original
definition, which is wrong (because it has no module).

E.g. field x_foo introduced in my_mod by extending model bom from
mrp, the resulting xmlid would be `field_mrp_bom__x_foo` when in reality
it should be `field_my_mod_bom__x_foo`, however this patch simply
disables the feature for custom fields as there is little to no use for
them to have an automatic xmlid.

Task-ID: 2025151

closes odoo/odoo#34265

Signed-off-by: Raphael Collet (rco) <rco@openerp.com>