CPack/DragNDrop: Re-implement SLA attachment to avoid deprecated tools
The Rez
tool has been deprecated since Xcode 6. The hdiutil flatten
and hdiutil unflatten
tools have been deprecated since macOS 10.15 and
are removed in macOS 11. Instead use hdiutil udifrez
to attach the
SLA resources to disk images. This tool accepts XML input files, so
convert our resource file generation to produce that format.
Fixes: #20889 (closed)