[refactor] move DoInit phases into functions

[MarlonGamez]

Related: #4915

Description
No funcional changes, just organizing some skaffold init related code into functions, which will be useful for later when implementing necessary changes for #4915

Follow-up Work (remove if N/A)
Implement flow for transparently running skaffold init when the user doesn't have a config

[codecov]

Codecov Report

Merging #5037 (ae71180) into master (7991add) will increase coverage by 0.09%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master    #5037      +/-   ##
==========================================
+ Coverage   72.18%   72.28%   +0.09%     
==========================================
  Files         365      368       +3     
  Lines       12861    12927      +66     
==========================================
+ Hits         9284     9344      +60     
- Misses       2886     2891       +5     
- Partials      691      692       +1     

Impacted Files	Coverage Δ
pkg/skaffold/initializer/init.go	58.92% <75.00%> (+7.76%)	⬆️
pkg/skaffold/docker/reference.go	100.00% <0.00%> (ø)
pkg/skaffold/docker/default_repo.go	100.00% <0.00%> (ø)
pkg/skaffold/schema/v2beta9/upgrade.go	100.00% <0.00%> (ø)
pkg/skaffold/kubernetes/manifest/images.go	100.00% <0.00%> (ø)
...g/skaffold/kubernetes/portforward/pod_forwarder.go	82.00% <0.00%> (ø)
...ffold/kubernetes/portforward/resource_forwarder.go	86.00% <0.00%> (ø)
...kubernetes/portforward/port_forward_integration.go	0.00% <0.00%> (ø)
pkg/skaffold/deploy/helm/helm.go
pkg/skaffold/deploy/helm/util.go	80.39% <0.00%> (ø)
... and 7 more

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[tejal29]

couple of nits