* Implement SourceUnitEnumChunker for GitHub
This change refactors the internal scan method to introduce a scanRepo
method to perform the actual scan.
* Export unit fields so the values are captured in the report
* Add comment for scanRepo
* Break out ensureRepoInfoCache into a method
* Update comments and check errors
* Ensure that the repoInfoCache contains the repo during ChunkUnit
* Add integration test for ChunkUnit
* Move s.scanOptions initialization to Init()