Why does Guix duplicate dependency versions from Cargo.toml?

[Jonas Møller]

Hi Guix! Why does cargo-build-system need #:cargo-inputs specified in the package definition? This seems like a big mistake for a couple of reasons.

1. It is completely redundant, it should match what is in Cargo.toml. I know `guix import crate` exists to automate this process, but I don't understand the rationale for duplicating this information.
   
2. It is bad practice for Guix to override Cargo.lock if it exists, this means that Guix is building a different binary to the one the developers of the packaged Rust application are seeing on their end, this is a much bigger problem.  This can and will cause spurious build failures, or bugs that are unknown to the developers of the Rust programs that Guix packages.