skip-build shouldn't detect builders

[balopat]

When --skip-build is passed in, skaffold init should just skip artifacts section in the skaffold.yaml. This works, however currently we still traverse into the subdirectories and detect all the builders! This PR removes that issue.

Note: There is no significant time diff on my laptop weirdly on the skaffold repo between master and this version. Maybe I'm missing something, or artifact detection is fast...

This also helps moving the design toward a visitor pattern, where I could turn on and off certain visitors that aggregate the state independently in the analysis struct.

[codecov]

Codecov Report

Merging #3528 into master will decrease coverage by <.01%.
The diff coverage is 50%.

Impacted Files	Coverage Δ
pkg/skaffold/initializer/init.go	29.12% <50%> (-0.29%)	⬇️