init --analyze should return unique image names

[caulagi]

Fixes #3128

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes unit tests
• Mentions any output changes.
• Adds documentation as needed: user docs, YAML reference, CLI reference.
• Adds integration tests if needed.

Reviewer Notes

• The code flow looks good.
• Unit test added.
• User facing changes look good.

Release Notes

Bug fixes
    * "Skaffold init --analyze returns a unique list of image names"

[codecov]

Codecov Report

Merging #3141 into master will increase coverage by 0.13%.
The diff coverage is 100%.

Impacted Files	Coverage Δ
pkg/skaffold/initializer/init.go	58.16% <100%> (+0.97%)	⬆️
pkg/skaffold/build/local/local.go	31.57% <0%> (-1.76%)	⬇️
pkg/skaffold/docker/remote.go	12.96% <0%> (-1.04%)	⬇️
pkg/skaffold/runner/diagnose.go	12.5% <0%> (-0.36%)	⬇️
pkg/skaffold/kubernetes/context/context.go	100% <0%> (ø)	⬆️
pkg/skaffold/schema/versions.go	70.83% <0%> (ø)	⬆️
pkg/skaffold/config/options.go	100% <0%> (ø)	⬆️
pkg/skaffold/schema/v1beta17/upgrade.go	77.77% <0%> (ø)
...ck/tools/vendor/github.com/rakyll/statik/statik.go	100% <0%> (ø)
pkg/skaffold/build/local/buildpacks.go	0% <0%> (ø)
... and 13 more

[balopat]

Thank you for opening this @caulagi!
I think we can simplify this - if you add the test in TestResolveBuilderImages and implement the "set semantics" for unresolvedImages via map[string]interface{} in autoSelectBuilders instead of the json printer methods. What do you think?

[dgageot]

Just a few nits

[caulagi]

@balopat, @dgageot Thanks for the feedback. Can you have one more look?

[balopat]

LGTM, please run a gofmt to fix the Travis Check

[caulagi]

Oops, fixed.

Addressed