`parseFileSource()` and `parseLiteralSource()` are utility functions.
We have a package already for utility functions, `kubectl/util/`.
Move utility functions to `kubectl/util`, capitalize function names to
export from package.
Remove kubectl's dependence on pkg/api/helper, as part of
broader effort to isolate kubectl from the rest of k8s.
In this case, the code becomes private to kubectl; nobody else uses it.
**Which issue this PR fixes**
Part of a series of PRs to address kubernetes/community#598
**Special notes for your reviewer**:
**Release note**:
```release-note
NONE
```
tcharding