refactor: remove unnecessarily long paths in imports and mocks (#14429)

lib/manager/buildkite/extract.spec.ts

lib/manager/bundler/artifacts.spec.ts

lib/manager/composer/artifacts.spec.ts

lib/manager/composer/utils.spec.ts

lib/manager/npm/post-update/lerna.spec.ts

lib/util/exec/buildpack.spec.ts

lib/util/exec/index.spec.ts

lib/util/fs/index.spec.ts

lib/util/git/conflicts-cache.spec.ts

lib/util/http/github.spec.ts

lib/util/http/github.ts

lib/workers/pr/changelog/github.spec.ts

lib/workers/pr/changelog/gitlab.spec.ts

lib/workers/repository/onboarding/branch/index.spec.ts