The `CPACK_EXTERNAL_PACKAGE_SCRIPT` script may set this list variable to the
full paths of generated package files. CPack copy these files from the stage
directory back to the top build directory and possibly produce checksum files
if the `CPACK_PACKAGE_CHECKSUM` is set.
Ext and External were used inconsistently in the code and the
docs. This change converts all uses of Ext to External, including
within variable names used by the generator.
CPACK_EXT_ENABLE_STAGING enables optional staging
and CPACK_EXT_PACKAGE_SCRIPT allows to specify an optional
script file that can package staged files via an
external packaging tool.
Issue: #18236
