Replace util.SyncStore implementation to use singleflight and sync.Map.

[tejal29]

This is follow up of #4896

In #4896, I used a library sync/singleflight to make sure getDependencies results were cached and only computed once for dockerfile builder.
sync/singleflight ensure same function for given input key executes only once across multiple goroutines at a given time. Along with sync.Map, we can make sure at any given time, the function for given input key only executes once across multiple subroutines.
This is ensured by, wrapping the result of the function we want to execute and storing it in a thread safe sync.Map
For cache miss, execute the func and store the result.

During review @gsquared94 mentioned, he added util.Store which uses sync.Once and sync.Map to achieve th same thing.

sync/singleflight is safer for a corner case when same key is looked up exactly at the same point in different subroutines.

Hence replacing the sync.Store implementation with sync/singleflight

No User facing changes

[codecov]

Codecov Report

Merging #5016 (5a1c85a) into master (f2879db) will increase coverage by 0.01%.
The diff coverage is 76.92%.

@@            Coverage Diff             @@
##           master    #5016      +/-   ##
==========================================
+ Coverage   72.15%   72.16%   +0.01%     
==========================================
  Files         365      365              
  Lines       12807    12837      +30     
==========================================
+ Hits         9241     9264      +23     
- Misses       2879     2884       +5     
- Partials      687      689       +2     

Impacted Files	Coverage Δ
pkg/skaffold/build/cache/hash.go	71.28% <50.00%> (-2.12%)	⬇️
pkg/skaffold/docker/dependencies.go	73.33% <77.77%> (-1.67%)	⬇️
pkg/skaffold/util/store.go	100.00% <100.00%> (ø)
pkg/skaffold/build/result.go	83.33% <0.00%> (-3.34%)	⬇️
pkg/skaffold/build/bazel/build.go	67.24% <0.00%> (-3.13%)	⬇️
pkg/skaffold/server/server.go	45.36% <0.00%> (ø)
pkg/skaffold/build/jib/maven.go	100.00% <0.00%> (ø)
pkg/skaffold/build/jib/gradle.go	100.00% <0.00%> (ø)
pkg/skaffold/color/formatter.go	100.00% <0.00%> (+10.00%)	⬆️

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