This is a Cargo subcommand which transitively downloads a project's dependencies.
Currently this can be installed with:
$ cargo install --git https://github.com/robbym/cargo-download-deps.git
$ cargo download-deps --download-path=/path/to/dest --config=/path/to/Cargo.toml
The Cargo.toml must have at least:
[package]
name = "dummy"
version = "0.0.0"
And an emptry ./src/lib.rs.