I’ve updated the patch series with the following improvements:

* I've applied any naming feedback which came from Ludo
* I've changed the cargo-build-system to only unpack inputs into the cargo
vendor directory if the following applies:
  - The input is a path to a gzip tarball
  - The archive contains a file called Cargo.toml at its root
  - This means that we no longer require crate sources to include a
  ".crate" extension

Whew, given that this has gotten pretty long, I'd be happy to land this as it
is for now (barring any blocking issues!), and iterating further going forward!

Thanks again to everyone for their feedback!
—Ivan