Show Less Stdout when using --imgpkg-lock-output

**Describe the problem/challenge you have**

When generating an ImagesLock file with `kbld -f ... --imgpkg-lock-output ...`, stdout showing the configs from `-f` is displayed as part of the output of running the command:

```
kbld -f package-contents/config/ --imgpkg-lock-output package-contents/.imgpkg/images.yml
resolve | final: docker.io/dkalinin/k8s-simple-app@sha256:4c8b96d4fffdfae29258d94a22ae4ad1fe36139d47288b8960d9958d1e63a9d0 -> index.docker.io/dkalinin/k8s-simple-app@sha256:4c8b96d4fffdfae29258d94a22ae4ad1fe36139d47288b8960d9958d1e63a9d0
---
simple-app: ""
---
apiVersion: v1
kind: Service
metadata:
name: simple-app
namespace: default
spec:
ports:
- port: null
targetPort: null
selector: null
---
apiVersion: apps/v1
kind: Deployment
metadata:
annotations:
kbld.k14s.io/images: |
null
name: simple-app
namespace: default
spec:
selector:
matchLabels: null
template:
metadata:
labels: null
spec:
containers:
- env:
- name: HELLO_MSG
value: null
image: index.docker.io/dkalinin/k8s-simple-app@sha256:4c8b96d4fffdfae29258d94a22ae4ad1fe36139d47288b8960d9958d1e63a9d0
name: simple-app
---
app_port: 80
hello_msg: stranger
svc_port: 80

Succeeded
```

It makes it difficult to know exactly what happened as far as producing the ImagesLock file.

**Describe the solution you'd like**

The output should not show the underlying Kubernetes manifests in stdout. There should maybe just be information detailing what image references were resolved to a digest format and something confirming the ImagesLock file was successfully created and is located where the user wants the file created.

**Anything else you would like to add:**

Open to any interpretation of what the output when using `--imgpkg-lock-output` should be, but my thought is at very least to remove what has been resolved via the `-f` option.

Would also be open to hear if there are use cases for producing this stdout with `--imgpkg-lock-output`. At very least, I do not think the default should show the resolved Kubernetes configs.

---
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