Support subprojects from repositories

[LPGhatguy]

Some projects don't have a tool as their primary project or might publish multiple tools from the same repository.

It would be useful to be able to declare dependencies on these, maybe like:

[tools]
genref = { source = "rojo-rbx/rbx-dom/generate-reflection", version = "1.2.6" }

We would probably need a special convention for tags to make this kind of detection feasible, like generate-reflection-v1.2.6. I've been following this in the rbx-dom repository and it seems to work fairly well.