flux tree: Add namespaces to objects reconciled from HRs

[stefanprodan]

Fix: #2149

Before:

$ flux tree ks apps-sync
Kustomization/flux-system/apps-sync
├── Namespace/apps
├── HelmRelease/apps/podinfo
│   ├── Service/podinfo
│   ├── Deployment/podinfo
│   └── HorizontalPodAutoscaler/podinfo
├── Ingress/apps/podinfo
└── HelmRepository/apps/podinfo

After:

$ flux tree ks apps-sync
Kustomization/flux-system/apps-sync
├── Namespace/apps
├── HelmRelease/apps/podinfo
│   ├── Service/apps/podinfo
│   ├── Deployment/apps/podinfo
│   └── HorizontalPodAutoscaler/apps/podinfo
├── Ingress/apps/podinfo
└── HelmRepository/apps/podinfo