After deleting the .m2 repo node org/eclipse, tried to build eclipse plugins and found the cas-editor was failing because it was trying to download a dependency on org.eclipse.equinox:app version 1.0.0. The one in the repo is at version 1.0.0-v20070606.
I vaguely recall some slight upgrade in more recent Maven impls that slightly changed the matching rules for these kinds of things. Anyways, it now doesn't match or work.
This wasn't noticed by me because I had never deleted the org/eclipse directory in my local maven repo before.
The fix seems to be simple: I added an explicit dependency in the POM for this using a version range which got that version with the -v20070606 suffix. Because it was there in the .m2 repo, some other part of maven dependency was successful in resolving it.