The HST addon-module loading mechanism uses JAXB to load specific module definitions into ModuleDefinition class instance.
For this mechanism an xsd is generated, every build, based on the ModuleDefinition JAXB annotations and thereafter incorporated into the build artifact.
However, the ModuleDefinition class hasn't been changed since its creation (CMS 7.7 timeframe), nor is it expected to be changed anytime soon.
So re-generating the xsd each and every build isn't really nessesary (and I'm not sure even if the xsd itself is that much useful), and better can be moved and checked into the source tree itself instead.
If/when the ModuleDefinition is changed or extended, then (once) generating an updated xsd locally by the developer should be pretty trivial, or even updating it by hand might be feasible, after which it again can be checked into the source tree, probably as a updated (newer) versioned xsd.