refactor: move semver to lib/versioning

lib/config/validation.js

lib/datasource/github.js

lib/manager/_helpers/node/package.js

lib/manager/bazel/package.js

lib/manager/buildkite/package.js

lib/manager/docker/package.js

lib/manager/npm/package.js

lib/manager/npm/versions.js

lib/manager/pip_requirements/package.js

lib/manager/travis/package.js

lib/util/package-rules.js

lib/versioning/semver/index.js

lib/workers/pr/changelog/manager-npm.js

lib/workers/pr/changelog/manager-pip.js

lib/workers/pr/changelog/source-github.js

test/util/semver.spec.js