Fix xsd 4.2.0 includes

[kwabenantim]

Supports #102

[kwabenantim]

This is now building correctly here: https://github.com/Chaste/dependency-modules/actions/runs/25383678359/job/74438671253

[Copilot]

Pull request overview

Updates the XSD install script to handle the XSD 4.2.0+ upstream distribution layout so the install no longer fails on outdated download URLs and so the runtime headers are exposed via module include paths.

Changes:

• Switch XSD 4.2.0+ downloads to the newer linux-glibc2.31 tarball location and add the separate libxsd download.
• Extract libxsd for 4.2.0+ and set an include_dir for both pre/post-4.2.0 layouts.
• Extend the generated modulefile to prepend include paths (INCLUDE, C_INCLUDE_PATH, CPLUS_INCLUDE_PATH) and validate the include directory in ModulesTest.

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