We've talked in the past about having version specific public IDs available
in the catalog, but we would also have the version-agnostic version. The ID
with no version would always point to the latest set of document types.
This allows users to update their DTDs without having to update the DOCTYPE
in their files.
Of course, that particular concern is only a worry for the actual DTD
files, not for the modules. So, I'm not sure if we want the modules to use
public IDs with versions. My guess would be no, for consistency... but are
there any other opinions?