[docbook-tc] Further modularization of DocBook hierarchy?

Currently, all of the DocBook hierarchy is in a single module -- the
dbhier.mod file. It seems like if we wanted to, we could further
modularize the hierarchy by grouping parts of it into sub-modules, in
separate files, that could each be included or ignored through use of
parameter entity switches.

That'd make it easier for DTD implementors to mix and match or ignore
whatever "building blocks" from the standard hierarchy they choose.

I've got specific ideas about how the hierarchy could be divided up,
but at this point, I just wanted to find out if anybody else on the
list thinks it'd be useful to do it all. Maybe the TC has already
discussed it as part of the parameter-entity reorganization plan.

  --Mike