image inspect fixes & cleanup

[apostasie]

Overall, this enhances image inspect:

• multiple images with the same digest are now coalesced properly into one, with multiple RepoTags
• multiple images resulting for multiple arguments in the call to image inspect will coalesce into one array, instead of a stream of arrays
• ImageFromNative has been cleaned-up and simplified, making room for later addition of currently missing properties

In the process, we stopped calling on the image Walker which serves little to no purpose here, hence significantly reducing the complexity.

This mainly fixes #3004 and part of #3006.

After this, more work is needed overall for inspect across objects, and then some specifically for image inspect.

[apostasie]

hey @AkihiroSuda

Most of my PRs are failing the CIs.
Looks like:

#85 1.141 cni-plugins-linux-amd64-v1.4.1.tgz: FAILED
#85 1.142 sha256sum: WARNING: 1 of 1 computed checksums did NOT match

If this is it ^, did the cni-plugins folks do something messy?

[AkihiroSuda]

Asking in:

• v1.4.1 binaries were modified? containernetworking/plugins#1038

[apostasie]

@AkihiroSuda I am closing this.

There is a lot more work needed to fix image inspect which I am currently working on.
I'll reopen a more thorough PR (hopefully with tests) once I am fully done.