After extensive experimentation and several conversations with Adobe support,
I am able to confirm that Tnorthwest was correct about the file system.
The package must be created AND run on an HFS volume. FAT32, ExFAT, NTFS all produce various cryptic and undefinable errors if you try to create packages on them.
I also got mysterious error messages if I tried to copy a finished package over to a different file system. I was under the impression a .pkg file was a file, not a folder, so maybe there's some pattern of bytes that only HFS+ can read? How does that happen? Ext3/Ext4 also don't work, so I am guessing this is some sort of bizarroland interaction between HFS and the Apple packager.
This was a very frustrating bug, I spent way too much time trying to figure out what the problem was.