Need to use Package.extras instead

ckan/ckan#8288

Move from an inheritance approach to a more "composition" one,
separating properties handling in three kinds of methods:

* `_base()` methods with common properties to *all* versions
* `_vX()` methods with properties also applied to higher versions
* `_vX_only()` methods with properties only applied to the current
  version

So for instance, the DCAT AP 3 profile calls the following methods in
order:

```
    def graph_from_dataset(self, dataset_dict, dataset_ref):

        # Call base method for common properties
        self._graph_from_dataset_base(dataset_dict, dataset_ref)

        # DCAT AP v2 properties also applied to higher versions
        self._graph_from_dataset_v2(dataset_dict, dataset_ref)

        # DCAT AP v2 scheming fields
        self._graph_from_dataset_v2_scheming(dataset_dict, dataset_ref)

        # DCAT AP v3 properties also applied to higher versions
        self._graph_from_dataset_v3(dataset_dict, dataset_ref)

```

Fix method call in dcat ap 3 profile, surfaced after switching the
default profile. Add changelog and update docs.