Better handling of imported xsds and namespace tag generation

[RemkoMolier]

These are my accumulated changes with regard to: xsdgen: imports (being relative) xsd: type handling (referencing elements should not get a default type) xsd/xsdgen: generation of namespace information in golang struct xml tags only in case there is a need (reference / form="qualified" or different namespace) xsdgen: generation of only these types that have been defined in the targetNamespace(s) provided