Fix issues in skaffold init

[dgageot]

• Close files
• Don't panic on random yaml files
• Parse skaffold configs only if they are yaml|yml|json files

[codecov]

Codecov Report

Merging #3840 into master will increase coverage by 0.01%.
The diff coverage is 75%.

Impacted Files	Coverage Δ
pkg/skaffold/schema/versions.go	75% <100%> (+1.92%)	⬆️
pkg/skaffold/docker/docker_init.go	100% <100%> (ø)	⬆️
pkg/skaffold/initializer/analyze/analyze.go	89.09% <100%> (ø)	⬆️
pkg/skaffold/initializer/init.go	37.2% <50%> (-1.43%)	⬇️
pkg/skaffold/kubernetes/util.go	87.3% <75%> (-2.36%)	⬇️
pkg/skaffold/docker/dependencies.go	75% <0%> (+1.08%)	⬆️