In general, I'm wary of tools that try to analyze dependency information on their own ("best-guess" dependency analysis). -MMD and /showIncludes are preprocessor directives, but they are usually used in the context of a full TU compilation (in the case of tools attempting to extract dependency information, such as Ninja). As far as I am aware, tools using these options for dependency information are already performing full semantic analysis as part of dependency extraction. I anticipate that it will be similar with modules.
Adding features to explicitly allow tools to do "best-guess" dependency analysis feels too close to promoting what *I* consider bad behavior, but I'm very opinionated on the subject. Others may have differing opinions.
In general, I'd rather make tools that behave more correctly (and facilitate the creation of those tools through language and library design) than to try and cater to tools that do things poorly.