imgpkg Refactoring

[danielhelfand]

This is a refactor of imgpkg to address the following issues:

• Removal of any unused code brought over from kbld
• Cleaning up the switch statement in GetUnprocessedURLs in cmd/copy.go to remove problems of DRY
• Moving ImagesLock and BundleLock into a separate package outside of cmd (Extract lock files to their own packages #17)
• Creation of the concept of a Bundle via a Bundle struct
• Switching the YAML dependency (Move to k8s yaml package #16)
• General cleanup around all imgpkg code (correcting typos, updating language in error messages, adding tests)

[joaopapereira]

Closes #17, #16

[joaopapereira]

The PR looks good. I added some minor changes, especially around the imports.
The Switch refractory looks good, nevertheless, it still feels a little big (I think we can address that in the future).
Personally, I would prefer to use fewer constants in our tests to ensure that the change I pointed out would be more easily detected.

[danielhelfand]

Just wanted to leave a comment here that perhaps it would make sense to discuss introducing this refactor after the release is cut.

[joaopapereira]

looks good to me

[pivotaljohn]

(git-history, if-feasible) Could we squash any commits that fall under the same intent?

[danielhelfand]

(git-history, if-feasible) Could we squash any commits that fall under the same intent?

Not sure if feasible, but can keep in mind for future prs to be more mindful of commit history.