Simplify schema versioning

[dgageot]

I tried to simplify the code as much as possible so that introducing new versions of the schema requires moving and copying fewer things around

Number of instances of v1alpha in the sources:
find . -name .go -not -path "./vendor/" -exec cat {} ; | grep -c v1alpha
BEFORE: 143
AFTER: 21

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

[codecov-io]

Codecov Report

Merging #1067 into master will increase coverage by 0.78%.
The diff coverage is 48.48%.

@@            Coverage Diff             @@
##           master    #1067      +/-   ##
==========================================
+ Coverage   41.22%   42.01%   +0.78%     
==========================================
  Files          72       71       -1     
  Lines        3134     3187      +53     
==========================================
+ Hits         1292     1339      +47     
- Misses       1710     1715       +5     
- Partials      132      133       +1

Impacted Files	Coverage Δ
pkg/skaffold/build/gcb/cloud_build.go	0% <ø> (ø)	⬆️
pkg/skaffold/build/local/docker.go	21.05% <ø> (ø)	⬆️
pkg/skaffold/build/local/local.go	62.5% <ø> (ø)	⬆️
pkg/skaffold/deploy/helm.go	63.31% <ø> (ø)	⬆️
pkg/skaffold/build/local/bazel.go	0% <ø> (ø)	⬆️
pkg/skaffold/docker/image.go	59.18% <ø> (ø)	⬆️
pkg/skaffold/deploy/kubectl/cli.go	0% <ø> (ø)	⬆️
pkg/skaffold/build/gcb/desc.go	100% <ø> (ø)	⬆️
cmd/skaffold/app/cmd/cmd.go	0% <ø> (ø)	⬆️
pkg/skaffold/deploy/kustomize.go	36.23% <ø> (ø)	⬆️
... and 17 more

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[balopat]

I really like the simplifications, thank you very much!