xsd-fu: Use package namespaces throughout and consolidate commands

- replace `opts.package` use with individual package namespaces (this should have been done earlier)
- add new `omexml_model_all` and `omexml_metadata_all` commands which generate all model and metadata code, respectively.  These replace the use of individual targets for ant, maven and cmake, reducing the number of xsd-fu invocations and model loads, speeding up the build and making the build logic simpler.
- don't hardcode package/namespaces into the templates; this was done for `MapPairs` and in some other places in the templates; use the correct package variables throughout
- C++ code uses absolute namespaces throughout (needed to fix hardcoded relative namespace usage)
- minor docstring corrections in templates (wrong package, outdated bits removed)

Testing: all builds should continue to work (ant, maven, cmake).  Verify that there are no breaking code changes [there are some namespace changes (made absolute), but no functional changes].

These changes are primarily to fix xsd-fu to not break when future changes are made by cleaning up accidental hardcoding of namespaces/packages.  And to improve build performance by consolidating multiple commands.  If we wanted, we could further consolidate them into a single omexml command to do all ome-xml source generation in a single shot.