De-duplicate images in final discovery output

[caxu-rh]

This change modifies DiscoveredImage to store the name/type of one or more containers which use the image, and it also adds a new Insert method on Manifest, which acts similarly to the built-in append except duplicate entries are merged into the existing DiscoveredImage instance in the list.

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A corresponding change is still needed on the productctl side to ingest the new schema.

[komish]

Couple of comments in-line. The only other thing I'll mention is that I'd like to keep the exported API surface to a minimum. The addition of methods on the exported structs makes them a part of the API surface. I'd say make the unexported methods unless absolutely necessary (here, just meaning - if you don't need them to enable other code within this repo, unexport them).

We can always promote them to exported at a later time.

Thanks for your work @caxu-rh

[caxu-rh]

I'd like to keep the exported API surface to a minimum.

Noted, I was able to change the receiver methods into regular, unexported methods instead (and removed functions for Manifest equality check and DiscoveredContainer equality check, which are no longer needed).

[komish]

/lgtm

[exe-prow-github-app]

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