bug#78431: Unclear patch submission address

The HACKING and README-hacking files contain advice to send patches to the
email address in the respective program's --help output.

However, --help output might not contain any email addresses, such as for
example in the coreutils that ships with Ubuntu 25.04. Looks as if both
PACKAGE_PACKAGER and PACKAGE_PACKAGER_BUG_REPORTS need to be set in order
to have at least one address there.

Maybe some rephrasing of the hacking docs or adding fallback info there in
case of no other email address source would be in order.

Ville