Test error when using reference field as _parent_ depends

This is a follup of !527 (comment 295958)

When adding the '_parent_shipment.carrier' fields on the depends I get the following traceback:

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ERROR: test_view (test_module.StockPackageShippingTestCase) (view=Pool().get('ir.ui.view')(321))
Test validity of all views of the module
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Traceback (most recent call last):
  File "/home/pokoli/projectes/tryton/trytond/trytond/tests/test_tryton.py", line 329, in test_view
    res = Model.fields_view_get(view_id)
  File "/home/pokoli/projectes/tryton/trytond/trytond/model/modelview.py", line 292, in fields_view_get
    result['arch'], result['fields'] = cls.parse_view(
  File "/home/pokoli/projectes/tryton/trytond/trytond/model/modelview.py", line 453, in parse_view
    fields_def = cls.__parse_fields(
  File "/home/pokoli/projectes/tryton/trytond/trytond/model/modelview.py", line 661, in __parse_fields
    fields_attrs = cls.__parse_fields(
  File "/home/pokoli/projectes/tryton/trytond/trytond/model/modelview.py", line 661, in __parse_fields
    fields_attrs = cls.__parse_fields(
  File "/home/pokoli/projectes/tryton/trytond/trytond/model/modelview.py", line 661, in __parse_fields
    fields_attrs = cls.__parse_fields(
  File "/home/pokoli/projectes/tryton/trytond/trytond/model/modelview.py", line 574, in __parse_fields
    views = get_views(relation, widget, view_ids, mode)
  File "/home/pokoli/projectes/tryton/trytond/trytond/model/modelview.py", line 544, in get_views
    view = Relation.fields_view_get(view_id=view_id)
  File "/home/pokoli/projectes/tryton/trytond/trytond/model/modelview.py", line 292, in fields_view_get
    result['arch'], result['fields'] = cls.parse_view(
  File "/home/pokoli/projectes/tryton/trytond/trytond/model/modelview.py", line 492, in parse_view
    fields2 = cls.fields_get(list(fields_def.keys()), level=level)
  File "/home/pokoli/projectes/tryton/trytond/trytond/model/model.py", line 204, in fields_get
    relation_fields = Relation.fields_get(level=level - 1)
  File "/home/pokoli/projectes/tryton/trytond/trytond/model/model.py", line 191, in fields_get
    definition[fname] = field.definition(cls, language)
  File "/home/pokoli/projectes/tryton/trytond/trytond/model/fields/function.py", line 175, in definition
    definition = self._field.definition(model, language)
  File "/home/pokoli/projectes/tryton/trytond/trytond/model/fields/field.py", line 528, in definition
    target = target._fields[field[8:]].get_target()
AttributeError: 'Reference' object has no attribute 'get_target'