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kubectl: Move utility functions to util package
**What this PR does / why we need it**:
`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.
**Special notes for your reviewer**:
**Release note**:
```release-note
NONE
```
/sig cli
/kind cleanup