refactor: git/storage -> git/index

lib/manager/composer/artifacts.spec.ts

lib/manager/gomod/artifacts.spec.ts

lib/manager/pipenv/artifacts.spec.ts

lib/platform/azure/index.spec.ts

lib/platform/azure/index.ts

lib/platform/bitbucket-server/index.spec.ts

lib/platform/bitbucket-server/index.ts

lib/platform/bitbucket-server/types.ts

lib/platform/bitbucket/index.spec.ts

lib/platform/bitbucket/index.ts

lib/platform/bitbucket/utils.ts

lib/platform/git/__snapshots__/index.spec.ts.snap

lib/platform/git/__snapshots__/storage.spec.ts.snap

lib/platform/git/index.spec.ts

lib/platform/git/index.ts

lib/platform/gitea/index.spec.ts

lib/platform/gitea/index.ts

lib/platform/github/index.spec.ts

lib/platform/github/index.ts

lib/platform/github/types.ts