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your .repo files and/or repo setup in /etc/yum.conf is broken
Copied /etc/yum directory from a working server, preserving the old one, and it seems to be working so far. It looks like the biggest difference is the old one had a conf file for fastestmirror, but the new one has installonlyn conf file. I'll see what happens. Maybe it munged when I added the downloadonly plugin earlier. I wanted to have all the updates on the machine before I ran the upgrade.