Move uploaded cargo bins to dist/ from cargo-dist

[brson]

Cargo uses its own dist directory, which currently isn't indexed. I'd rather just have it upload to the same place as the rest of rust. Requires updating rust-packaging.

[brson]

In addition to the necessary changes in rust-buildbot, this will also require modifying rust-packaging to download from the correct place, and (I think) the Cargo build system to download its snapshots from the new location.

[brson]

Closing because what we really want is to redesign the entire build process.