use a better name for rust-lang/rust checkout directory

[tshepang]

Instead of rustc-checkout, am thinking we use something more general, like just rust, because that repo has a lot more than Rust compiler code.

[Binlogo]

Inspired by this discussion, taking this a step further, could ⁠josh-sync evolve into a general repository synchronization tool? If all upstream and sub-repositories could be described within a structured ⁠josh-sync.toml config file, this would present a very flexible and powerful solution.

This is not an request, just an idea. It seems that josh-sync is a better solution than the bare git subtree.

[Kobzol]

It can already be used like that, outside of hard-coding rust-lang/rust as the parent repository (which can be easily changed), I don't think that there's much Rust-specific code in josh-sync.