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Macro to walk the module tree and generate mainargs entrypoints for any Task.Command methods that it finds.

Note that unlike the rest of Mill's module-handling logic which uses Java reflection, generation of entrypoints requires typeclass resolution, and so needs to be done at compile time. Thus, we walk the entire module tree, collecting all the module Class[_]s we can find, and for each one generate the mainargs.MainData containing metadata and resolved typeclasses for all the Task.Command methods we find. This mapping from Class[_] to MainData can then be used later to look up the MainData for any module.