The reason is that we can not expect the extended logic run on git
dependencies starting from Cargo 1.68 to be reproducible in future
versions, and thus the output hash would not be sufficiently stable.
https://github.com/rust-lang/cargo/pull/11414
- `toRustTarget` and friends pulled out from rust tools into rust
library. Since they don't depend on any packages they can be more
widely useable.
- `build-rust-package` gets its own directory
- `fetch-cargo-tarball` gets its own directory