refactor: add cargo skeleton

lib/config/definitions.js

lib/manager/cargo/artifacts.js

lib/manager/cargo/extract.js

lib/manager/cargo/index.js

lib/manager/cargo/update.js

lib/manager/index.js

test/manager/cargo/__snapshots__/artifacts.spec.js.snap

test/manager/cargo/artifacts.spec.js

test/manager/cargo/extract.spec.js

test/manager/cargo/update.spec.js

...rkers/repository/extract/__snapshots__/index.spec.js.snap