add command to copy bundle referenced images to registry where bundle already resides

**Describe the problem/challenge you have**

- i use dockerhub registry
- i create bundle in my dockerhub account (that references images from all over the internet)
- i dont want to "copy" my bundle to a different location just to ensure that referenced images are copied into my docherhub account

**Describe the solution you'd like**

currently one would have to do the following:

- imgpkg push -b docker.io/dkalinin/foo:v1.0.0 -f my-bundle/
- imgpkg copy -b docker.io/dkalinin/foo:v1.0.0 --to-repo docker.io/dkalinin/foo:v1.0.0-copied (not sure if i can just use same tag?)

this seems a bit repetitive. proposed steps:

- imgpkg push -b docker.io/dkalinin/foo:v1.0.0 -f my-bundle/
- imgpkg copy -b docker.io/dkalinin/foo:v1.0.0 --to-same-bundle (or may be a better name for the flag)

i think having an explicit flag like --to-same-bundle is good to avoid confusion of now specifying anything for other from-to pairs.

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

this "promotes" the use case for ensuring i have all materials for the bundle in my registry to first class.

this might be also useful to "repair" bundle in your registry if some references images got deleted.