refactor: move util/exec to own folder (#5125)

lib/manager/bundler/artifacts.ts

lib/manager/cargo/artifacts.ts

lib/manager/composer/artifacts.ts

lib/manager/gomod/artifacts.ts

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

lib/manager/pipenv/artifacts.ts

lib/manager/poetry/artifacts.ts

lib/util/exec/docker/index.ts

lib/util/exec/env/basic-vars.ts

lib/util/exec/env/index.ts

lib/util/exec/index.ts

test/manager/bundler/artifacts.spec.ts

test/manager/cargo/artifacts.spec.ts

test/manager/composer/artifacts.spec.ts

test/manager/gomod/artifacts.spec.ts

test/manager/gradle/index.spec.ts

test/manager/mix/artifacts.spec.ts

test/manager/pip_setup/extract.spec.ts

test/manager/pip_setup/index.spec.ts

test/manager/pipenv/artifacts.spec.ts