There is nothing special about unpacking an .mpkg or .pkg installer – in terminal mode you see they are just directories. Just right click on them and select “show archive contents”…
Later on 7-zip will help you with unpacking the contained archives inside of those .mpkg and .pkg files…
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Doesn’t work, not accurate at all
After checking with the ‘file’ command, .mpkg files appear to be ‘xar’ archives, and can be extracted (and presumably created) with the ‘xar’ command from the terminal. E.g. “xar -xf myInstallationPackage.xar”