Support platform specific dependencies

[wildarch]

Add select({})s to deps attributes that select dependencies based on the rules_rust target platform matched. This allows generating a single cargo_dependencies.bzl file that can be used for any target triple.

For now we only support 64-bit Linux and Mac. Other targets should work too, but we cannot test them with the currrent CI setup.

Builds on https://github.com/SirWindfield/blackjack/pull/1 to retrieve the correct cargo executable on all platforms and pass the cargo executable and manifest paths in args.

I would like to add support for windows too (and got pretty far into supporting that), but ran into some issues with winapi, which is hard for me to debug without access to a Windows machine outside of CI.

[wildarch]

@sirwindfield this approach seems to work quite well, do you think it covers your needs for #5?

[mainrs]

Looks good! What problems did you encounter when trying to add Windows?

[wildarch]

Looks good! What problems did you encounter when trying to add Windows?

The winapi crate was giving me some issues. I think it's just about enabling the right flag (looks like normally the build script takes care of that), but without an interactive windows environment to test I couldn't get things to build. If someone has access to a Windows machine and can get this sorted, that would be awesome 😊

[mainrs]

google/cargo-raze#41 contains a good amount of code related to special crates. Might come handy :)