Automatic merge from submit-queue (batch tested with PRs 51047, 48573, 50764, 51092, 50578)
Library updates to better support removing kubectl deps on kubernetes/kubernetes
**What this PR does / why we need it**:
**Which issue this PR fixes**: kubernetes/kubectl#50
**Special notes for your reviewer**:
**Release note**:
```release-note
NONE
```
**What this PR does / why we need it**:
Makes functions in validation/schema.go private to kubectl,
further isolating kubectl.
**Which issue this PR fixes**
Part of a series of PRs to address kubernetes/community#598
**Release note**:
```release-note
NONE
```
**What this PR does / why we need it**:
Visibility rules allow dependency control. The rules currently in
place make all targets public, to override the default state of
private. This PR removes public visibility from kubectl code. It
uses specially named pacakge groups to identify "bad" dependencies on
kubectl code for later refactoring or removal.
**Which issue this PR fixes**
First in a series of PRs to address kubernetes/community#598
**Release note**:
```release-note
NONE
```
