DRILL-6515: Render a link between the Unnest operator and it's source
* DRILL-6515: Render a link between the Unnest operator and it's source
1. Unnest operator is expected to specify srcOp=##-## to help identify the source of its input.
2. Implicit Source Map is leveraged to capture implicit data flow pipelines between Unnest and Lateral. (Can be expanded to other operators too).
3. In addition, code refactored for more readability.
* Minor comment updates
* Minor Update
Converted implicitSrc to a local variable
closes #1327
