cargo.inc: switch to cargo_util.bbclass
cargo.inc: switch to cargo_util.bbclass

This does all the fetching through bitbake which allows it to be built
correctly in firewalled environments.

meta-rust/meta-rust,cardoe/meta-rust,starlab-io/meta-rust,meta-rust/meta-rust,meta-rust/meta-rust,jmesmon/meta-rust,cardoe/meta-rust,jmesmon/meta-rust,meta-rust/meta-rust,cardoe/meta-rust,jmesmon/meta-rust,jmesmon/meta-rust,starlab-io/meta-rust,cardoe/meta-rust,meta-rust/meta-rust