Add warning when trying to generate a HTML publication with modules, but all-modules-page-plugin is not in the plugin classpath

[aSemy]

Currently when a HTML Publication has multiple modules but the all-modules-page-plugin is missing, then Dokka will happily run and report no errors.

This is incredibly frustrating to fix! It's really baffling how everything looks like it's set up correctly, but the generated Publication is empty.

It should be quite simple to create a check to verify that if a Publication has multiple modules then all-modules-page-plugin should be present, otherwise, log an informative message.

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