depends: Switch to `cargo vendor` for Rust dependencies
When we first integrated Rust into our build system, we had two
limitations:
- We were building the `librustzcash` FFI library as a dependency, and
therefore needed access to its crate dependencies in the depends
system.
- Gitian builds happen offline, so we needed to fetch any crate
dependencies ahead of time, and then configure cargo to use these in
an offline environment.
At the time, `cargo` already had support for "Source Replacement", but
there was no easy way to package the dependencies in the necessary way.
What we implemented was effectively the `cargo-vendor` tool, built using
Makefiles. A noticeable downside was that we were pinning dependencies
twice: once in the `Cargo.lock` for the FFI library, and again in our
depends system.
Since then, `cargo-vendor` has been upstreamed into `cargo` itself, and
we have moved `librustzcash` into this repository. We can therefore use
`cargo vendor` directly from our pinned Rust compiler to fetch the
dependencies, and rely on our local `Cargo.lock` to pin the specific
crates we are relying on.
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