Account for nested imgpkg bundles in the caching logic

[100mik]

Describe the problem/challenge you have
imgpkg bundles might contain references to other imgpkg bundles (as in the case of PackageRepository bundles). Purely SHA based caching might lead to nested content being pulled from an older registry when a migration is attempted.

Relevant bug in kapp-controller: carvel-dev/kapp-controller#1603

Describe the solution you'd like
Instead of accounting for just the SHA as a key while caching content, the entire registry URL should be considered.

Anything else you would like to add:
This would not be an issue in a kapp-controller upgrade scenario as an update would roll out new pods, dropping the old cache.

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