Move code to get_tables_to_repack_partitioned

Move code to get_tables_to_repack_partitioned

Some of its code was pointlessly in its caller.  This makes it better
contained and clearer.

Backpatch to 19, to avoid having two different copies in case we have to
modify it again later.

Author: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Reviewed-by: ChangAo Chen <cca5507@qq.com>
Discussion: https://postgr.es/m/alD9l-XlCuu3eUEe@alvherre.pgsql