Archive extra files, relative to paths other than the target directory

[sunshowers]

In particular, we probably also want to add support for extra paths relative to the base output directory (e.g. target/debug, target/x86_64-unknown-linux-gnu/debug).

One challenge is that there can be multiple base output directories, and we may need a way to spec that out. (We don't currently support multiple --target options (#537), but upstream Rust does. Supporting multiple --targets isn't a requirement here, but we should ensure that whatever UI we come up with doesn't make supporting that harder.)