refactor: rename lock files -> artifacts

lib/config/definitions.js

lib/manager/cargo/artifacts.js

lib/manager/composer/artifacts.js

lib/manager/gomod/artifacts.js

lib/manager/npm/post-update/index.js

lib/manager/pipenv/artifacts.js

lib/workers/branch/commit.js

lib/workers/branch/get-updated.js

lib/workers/branch/index.js

lib/workers/pr/index.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/__snapshots__/get-updated.spec.js.snap

test/workers/branch/commit.spec.js

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

test/workers/branch/index.spec.js

...orkers/branch/lock-files/__snapshots__/index.spec.js.snap

test/workers/branch/lock-files/index.spec.js