Recursively build and diff Kustomizations

Signed-off-by: Boris Kreitchman <bkreitch@gmail.com>

cmd/flux/testdata/diff-kustomization/diff-with-recursive.golden

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+📁 Kustomization/default/my-app changed
+► ConfigMap/default/my-app created

cmd/flux/testdata/diff-kustomization/my-app.yaml

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+---
+apiVersion: kustomize.toolkit.fluxcd.io/v1
+kind: Kustomization
+metadata:
+  labels:
+    kustomize.toolkit.fluxcd.io/name: podinfo
+    kustomize.toolkit.fluxcd.io/namespace: {{ .fluxns }}
+  name: my-app
+  namespace: default
+spec:
+  interval: 5m0s
+  path: ./my-app
+  force: true
+  prune: true
+  sourceRef:
+    kind: GitRepository
+    name: podinfo
+  targetNamespace: default