Node.js v6 was end-of-life'd in April, 2019, with AWS Lambda prohibiting updaets to the Node.js v6 runtime since June 30, 2019.
This makes it quite safe for us to remove the Node 6 support from the repository.
SourceFactory was meant to cache Sources so that they could be used
in different preprocessor tasks.
This turned out to be over-engineering. This patch kills the layer.