Better check for valid Kubernetes manifests

[dgageot]

Old code: checks the file's extension first and later parse those files.
New code: checks the file's extension first and parse those files early.

This allows yaml/json files that are not valid kubernetes manifest to be analyzed as build descriptors. For example package.json

Signed-off-by: David Gageot david@gageot.net

[codecov]

Codecov Report

Merging #3404 into master will decrease coverage by 0.04%.
The diff coverage is 100%.

Impacted Files	Coverage Δ
pkg/skaffold/initializer/util.go	100% <ø> (ø)	⬆️
pkg/skaffold/initializer/kubectl/kubectl.go	83.33% <100%> (+1.24%)	⬆️
pkg/skaffold/initializer/init.go	60.61% <100%> (-0.13%)	⬇️
pkg/skaffold/util/tar.go	52.87% <0%> (-4.6%)	⬇️