v1 lock files (generated by default by Cargo versions 1.40 and below)
use a single table, `metadata`, to store the checksums of packages.
The primary motivation for doing this now is that we're considering
vendoring all Cargo lock files in Nixpkgs, some packages still use it
(e.g. cargo-asm), and adding support for it doesn't increase the
complexity of the function. No matter the outcome of the vendoring
discussion, this is a nice thing to have because Cargo still supports v1
lock files.
