[init refactor] cleanup on analyzers and moving things into a single package

[balopat]

No functional change, pure refactor.

• moved kubectl package back into initializer - everything is unexported - we really don't need exporting there
• moved analyzers to their corresponding files - this way logic stay closer to each other - we can already see much easier how the kubectlnitializer gets the manifests - there are more simplifications to come there!

[codecov]

Codecov Report

Merging #3538 into master will increase coverage by 0.03%.
The diff coverage is 88.23%.

Impacted Files	Coverage Δ
pkg/skaffold/initializer/init.go	0% <0%> (ø)	⬆️
pkg/skaffold/initializer/config.go	100% <100%> (ø)	⬆️
pkg/skaffold/initializer/util.go	100% <100%> (ø)	⬆️
pkg/skaffold/initializer/builders.go	93.44% <100%> (+0.58%)	⬆️
pkg/skaffold/initializer/analyze.go	82.35% <60%> (-6.74%)	⬇️
pkg/skaffold/initializer/kubectl.go	87.34% <87.5%> (ø)
...affold/kubernetes/portforward/kubectl_forwarder.go	68.29% <0%> (+2.43%)	⬆️