Rustbuild doesn't handle multiple versions of crates.io crates in use

[alexcrichton]

Rustbuild compiles the compiler in a particular location and is then responsible for copying artifacts over to the actual structure the compiler is expecting (e.g. rustbuild assembles a sysroot). In order to handle stale folders rustbuild copies only the most recent version of each rlib, deduplicated by hash.

This pretty obviously, though, doesn't work with multiple versions of the same library! We should probably call Cargo with --message-format=json and learn about compiled artifacts, that way we can copy precisely what we need to into the sysroot.