pub fn from_unknown(base: &AbsoluteSystemPath, unknown: impl Into<Utf8PathBuf>) -> Self {
        // we have an absolute system path and an unknown kind of system path.
        let unknown: Utf8PathBuf = unknown.into();
        if unknown.is_absolute() {
            Self(unknown)
        } else {
            Self(
                base.as_path()
                    .join(unknown)
                    .as_std_path()
                    .clean()
                    .try_into()
                    .expect("clean should produce valid UTF-8"),
            )
        }
    }
Takes in a system path of unknown type. If it's absolute, returns the
path, If it's relative, appends it to the base after cleaning it.
crates/turborepo-paths/src/absolute_system_path_buf.rs