refactor: getArtifacts returns an array

lib/manager/bundler/artifacts.js

lib/manager/cargo/artifacts.js

lib/manager/composer/artifacts.js

lib/manager/gomod/artifacts.js

lib/manager/pipenv/artifacts.js

lib/workers/branch/get-updated.js

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

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

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

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

test/workers/branch/get-updated.spec.js