cargo: auto-fetch default sources file from rust-lang.org

[graydon]

We should be publishing a "default" local-sources.json file on rust-lang.org (or, at minimum, our github repo for now). When you run cargo for the first time it should fetch this if it hasn't got a local-sources.json file, and there should be a "cargo init" command or something that re-fetches this default.

[elly]

https://github.com/graydon/rust/pull/1363 handles this.

[brson]

Looks like it doesn't auto-fetch yet.