Move cudf._lib.copying to cudf.core._internals (#17548)

Contributes to #17317

Also I found that `PackedColumns` was not being use anywhere. It appears it was added back in #8153 for dask_cudf but I cannot see it being used there anymore

Authors:
  - Matthew Roeschke (https://github.com/mroeschke)

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

URL: #17548

Author: mroeschke

python/cudf/cudf/_lib/CMakeLists.txt

@@ -12,7 +12,7 @@
 # the License.
 # =============================================================================
 
-set(cython_sources column.pyx copying.pyx groupby.pyx interop.pyx scalar.pyx stream_compaction.pyx
+set(cython_sources column.pyx groupby.pyx interop.pyx scalar.pyx stream_compaction.pyx
                    string_casting.pyx strings_udf.pyx types.pyx utils.pyx
 )
 set(linked_libraries cudf::cudf)

python/cudf/cudf/_lib/__init__.py

@@ -2,7 +2,6 @@
 import numpy as np
 
 from . import (
-    copying,
     groupby,
     interop,
     stream_compaction,