add menuinst JSON file validation

[jaimergp]

This adds the logic to validate the new menuinst JSONs at build-time. It will only print a warning for now. If an error should be raised, we need to add a logic similar to the overlinking analysis, but I don't know if that's what people want... For now I kept it simple. Let me know!

[jaimergp]

Turns out utils.get_logger() injects a deduplication filter by default, so the 2nd test would never raise a warning because the message (albeit with different args!) had already been 'seen'. 🤷 dedupe=False is enough to make it work.

[jaimergp]

Errors unrelated to this PR (to be fixed by #5059).