Fix SearchFilter crash when search_fields uses the pk alias

[mohamed-alired]

Description

SearchFilter.construct_search() resolves the pk alias to the model's primary key
field, so search_fields = ['pk'] (or '=pk', or a lookup through a relation such as
'author__pk') builds a valid query. But must_call_distinct(), which runs right after
to decide whether duplicates are possible, passes 'pk' straight to
Options.get_field(), so the request fails with FieldDoesNotExist and a 500:

class UserList(generics.ListAPIView):
    queryset = User.objects.all()
    serializer_class = UserSerializer
    filter_backends = [filters.SearchFilter]
    search_fields = ['=pk']

GET /users/?search=1  ->  FieldDoesNotExist: User has no field named 'pk'

This resolves the alias in must_call_distinct() the same way construct_search()
does. Tests cover pk on the model, through a foreign key, through a many-to-many
(where distinct is still required, and a search by pk through a view. All three fail
without the fix.