Re-add the old feature, that -set.xml files can explicitly opt-out of
being included in the catalog. Useful for some development and helper /
included files in some aircraft.
Support multiple primary aircraft in a package, which has several
aircraft developers have requested. This adjusts the handling of
the variant-of and primary-set tags.
Additionally, this adds partial unit-test coverage for -set.xml
scanning and catalog XML generation.
Run ./test_catalog to run unit-tests of the catalog generation.
