(Only for feedback) +str #15977 Rename and change types in the new DSL

There are still a lot of TODOs here (especcially documentation), but I would like som feedback on what has been done.

* Clean up type signatures
* Chang names of Source/Sink/ProcessorFlow to Faucet/Drain/Pipe
* Introduce new Source/Sink/Flow
* Make all Pipe types package private
* Change the TypeGraph DSL to work on Flow/Sink/Source
I think `connect` is more embedded in the domain (of flowing water), `append` is more appropriate to the internal AST structure. But my preference is not very strong.