Consider how to handle camino::Utf8PathBuf

[illicitonion]

oli-obk/cargo_metadata#152 updated cargo_metadata to use camino::Utf8PathBuf instead of std::path::PathBuf. The idea is that, as cargo only supports UTF-8 paths, this simplifies some handling (e.g. Utf8PathBuf can impl Display, rather than needing .to_string_lossy() and .display() calls)

We should work out whether we want to treat this as a localised change in that API (converting back to PathBuf/Path on the boundary with cargo_metadata), or whether we want to adopt Utf8PathBuf ourselves.