init refactor/clearer k8s manifest parsing

[balopat]

small refactoring, cleanup in parsing k8s yamls in skaffold init.
The main changes:

• introduce yamlObject type alias for readability
• split out image parsing from yaml parsing into a single responsibility function parseKubernetesObjects and then using that in Kubernetes manifest file detection instead of image parsing

[codecov]

Codecov Report

Merging #3531 into master will increase coverage by <.01%.
The diff coverage is 95.83%.

Impacted Files	Coverage Δ
pkg/skaffold/util/util.go	86.48% <100%> (ø)	⬆️
pkg/skaffold/deploy/kubectl.go	65.87% <100%> (ø)	⬆️
pkg/skaffold/initializer/kubectl/kubectl.go	84.61% <95.45%> (+1.28%)	⬆️
pkg/skaffold/initializer/init.go	29.12% <0%> (-0.29%)	⬇️