fix(importer): handle buildkit index as manifest

mentioned in commit 8bf27fd7

mentioned in merge request gitlab-org/build/CNG!1023 (merged)

mentioned in merge request gitlab-org/charts/gitlab!2574 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1791 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1792 (merged)

mentioned in merge request omnibus-gitlab!6181 (merged)

mentioned in merge request gitlab-development-kit!2656 (merged)