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
Bartosz Kosiorek