Support for Mill `package.mill` files

[lefou]

Mill 0.12 projects can be split into sub-packages (package.mill files in sub-dirs). It would be nice if the Github Dependency Graph would be able to show the exact file location where a dependency is coming from.

[ckipp01]

This is a nice idea that I've thought about before. I honestly don't really know how to correct map them from Dep -> Location since when I pull the deps from coursier they don't have any of that information left. I'll need to think a bit on how to achieve this.