Move DetectWSL function into util package

[gsquared94]

Moved DetectWSL function into util package. This is a prereq for #4701

[codecov]

Codecov Report

Merging #4721 into master will increase coverage by 0.00%.
The diff coverage is 45.45%.

@@           Coverage Diff           @@
##           master    #4721   +/-   ##
=======================================
  Coverage   73.42%   73.42%           
=======================================
  Files         342      343    +1     
  Lines       13529    13560   +31     
=======================================
+ Hits         9934     9957   +23     
- Misses       2976     2980    +4     
- Partials      619      623    +4     

Impacted Files	Coverage Δ
pkg/skaffold/docker/client.go	79.38% <0.00%> (+2.02%)	⬆️
pkg/skaffold/util/wsl.go	55.55% <55.55%> (ø)
pkg/skaffold/deploy/kpt.go	82.85% <0.00%> (-2.47%)	⬇️
pkg/skaffold/kubernetes/owner.go	87.50% <0.00%> (ø)
pkg/skaffold/kubernetes/watcher.go	87.50% <0.00%> (ø)
pkg/skaffold/build/cluster/kaniko.go	0.00% <0.00%> (ø)
pkg/webhook/kubernetes/deployment.go	0.00% <0.00%> (ø)
pkg/skaffold/kubernetes/client.go
pkg/skaffold/kubernetes/client/client.go	0.00% <0.00%> (ø)

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