refactor: use manager subdirectory for grouping (#1040)

lib/config/presets.js

lib/manager/docker/registry.js

lib/manager/npm/registry.js

lib/workers/package-file/index.js

lib/workers/package/docker.js

lib/workers/package/npm.js

test/config/presets.spec.js

test/manager/docker/registry.spec.js

test/manager/npm/__snapshots__/registry.spec.js.snap

test/manager/npm/registry.spec.js

test/workers/package/docker.spec.js

test/workers/package/npm.spec.js

test/workers/repository/apis.spec.js