Decouple visiting manifests and replacing images

[dgageot]

This will be useful to apply more transformations to the manifests.
Signed-off-by: David Gageot david@gageot.net

[codecov-io]

Codecov Report

Merging #909 into master will increase coverage by 0.57%.
The diff coverage is 96.82%.

@@            Coverage Diff             @@
##           master     #909      +/-   ##
==========================================
+ Coverage   37.45%   38.02%   +0.57%     
==========================================
  Files          58       60       +2     
  Lines        2590     2609      +19     
==========================================
+ Hits          970      992      +22     
+ Misses       1509     1507       -2     
+ Partials      111      110       -1

Impacted Files	Coverage Δ
pkg/skaffold/deploy/kubectl/manifests.go	20.83% <ø> (-43.88%)	⬇️
pkg/skaffold/deploy/kubectl/images.go	100% <100%> (ø)
pkg/skaffold/deploy/kubectl/visitor.go	92.85% <92.85%> (ø)

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[balopat]

very nice, thank you! LGTM