Move the namespace selector code to package webhook/namespace
Move the conversion related code to package to webhook/versioned
Move errors related code to package webhook/errors
Move admission review related code to package webhook/request
Kubernetes-commit: 51774697b35314b078270e9da24fbe0ff843b981
Moved client and kubeconfig related code to webhook/config;
Moved the rule matcher to webhook/rules;
Left TODOs saying we are going to move some other common utilities;
Other code is moved to webhook/validation.
Kubernetes-commit: 1adfacc7eb41da109e970a9c2985fd55b4cbbdfd
Chao Xu