cargo.eclass: add cargo_live_src_unpack()

cargo.eclass: add cargo_live_src_unpack()

This function will allow using 'cargo fetch' during src_unpack
Since only new cargo supports vendoring, all live packages will
have to depend on >=rust-1.37.0

This enables us to ship live rust packages,
cargo fetch will download all crates and vendor them for offline
phases.

here's an example of src_unpack()

src_unpack() {
        if [[ "${PV}" == *9999* ]]; then
                git-r3_src_unpack
                cargo_live_src_unpack
        else
                cargo_src_unpack
        fi
}

Signed-off-by: Georgy Yakovlev <gyakovlev@gentoo.org>