[Modules] Dependency Generation

Hi, Thorsten,

For now, with Clang, I don't think there is a way to get dependency information from the compiler in a way akin to -MM. I'm not sure what Build2 is doing (the one public build system that's got some support for modules) - maybe it's doing its own scan of the source files looking for import lines.

The author of build2 wrote about this some time ago in a blog entry [1] (the last section of the entry is relevant here). The blog entry advocates a `-MM` for modules.

Regards,
Julian

[1] https://build2.org/article/cxx-modules-misconceptions.xhtml