[refactor] Move tag package outside of build

[nkubala]

this code does not have anything inherently to do with build, so it doesn't need to be nested under the pkg/skaffold/build package. let's move it out.

this is a non-functional refactor.

[codecov]

Codecov Report

Merging #5547 (42e2307) into master (161a30e) will decrease coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5547      +/-   ##
==========================================
- Coverage   71.29%   71.21%   -0.09%     
==========================================
  Files         400      400              
  Lines       14877    14934      +57     
==========================================
+ Hits        10607    10635      +28     
- Misses       3492     3511      +19     
- Partials      778      788      +10     

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/util.go	77.77% <ø> (ø)
pkg/skaffold/build/build.go	85.71% <ø> (ø)
pkg/skaffold/build/builder_mux.go	34.37% <ø> (ø)
pkg/skaffold/build/cache/lookup.go	80.00% <ø> (ø)
pkg/skaffold/build/cache/retrieve.go	65.62% <ø> (ø)
pkg/skaffold/build/cache/types.go	100.00% <ø> (ø)
pkg/skaffold/build/scheduler.go	94.33% <ø> (ø)
pkg/skaffold/initializer/build/util.go	100.00% <ø> (ø)
pkg/skaffold/runner/build_deploy.go	69.23% <ø> (ø)
pkg/skaffold/runner/new.go	63.13% <ø> (-4.46%)	⬇️
... and 15 more

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