feat(handlers): import route: return 409 conflict if repository is importing

mentioned in commit 1cfed781

mentioned in merge request gitlab-org/build/CNG!889 (merged)

mentioned in merge request gitlab-org/charts/gitlab!2386 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1513 (merged)

mentioned in merge request omnibus-gitlab!5899 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1514 (merged)

mentioned in merge request gitlab-development-kit!2421 (merged)

mentioned in merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1518 (merged)