Bug 23949 has been around for a long time, but it can finally be closed soon. I will be sending a last rites email for app-xemacs/xemacs-packages-sumo shortly, and after that the package will be moved into package.mask. If you want to easily install all xemacs packages with a single command, just use app-xemacs/xemacs-packages-all instead. You will get the same thing as is currently shipped in the sumo packages, but without all the file collisions that may occur.
I have also updated the XEmacs project documentation to reflect this change.