In #Rustlang you can easily use local packages. Specify a path in Cargo.toml:
[dependencies]
some_crate = { path = "/dir/of/some/crate/" }
The path can be relative. If it's a subdir, it will make a workspace: http://doc.crates.io/manifest.html#the-workspace-section (great for monorepos).
You can also use { git = "URL" }
http://doc.crates.io/specifying-dependencies.html#specifying-dependencies-from-git-repositories