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.
Set progress callback on cmake instance used by CPack. The progress
callback is used to output STATUS messages which are otherwise missing
when issued from e.g. CPack External staging scripts.
Fixes: #18567
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.
Alex Turbov