Avoid circular cimports in _lib/cpp/reduce.pxd (#14125)

This Cython modules contains some cimports from higher-level modules than it should, which introduces the possibility for circular import issues. Also it contains an unused import of DeviceScalar that can cause similar issues.

Authors:
  - Vyas Ramasubramani (https://github.com/vyasr)

Approvers:
  - Lawrence Mitchell (https://github.com/wence-)

URL: #14125

python/cudf/cudf/_lib/cpp/reduce.pxd

@@ -1,14 +1,13 @@
-# Copyright (c) 2020-2022, NVIDIA CORPORATION.
+# Copyright (c) 2020-2023, NVIDIA CORPORATION.
 
 from libcpp.memory cimport unique_ptr
 from libcpp.utility cimport pair
 
-from cudf._lib.aggregation cimport reduce_aggregation, scan_aggregation
+from cudf._lib.cpp.aggregation cimport reduce_aggregation, scan_aggregation
 from cudf._lib.cpp.column.column cimport column
 from cudf._lib.cpp.column.column_view cimport column_view
 from cudf._lib.cpp.scalar.scalar cimport scalar
 from cudf._lib.cpp.types cimport data_type
-from cudf._lib.scalar cimport DeviceScalar
 
 
 cdef extern from "cudf/reduction.hpp" namespace "cudf" nogil: