(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
this reads a little weird; what about removing `append/prepend` and only having `connect` which connects the pieces left to right (i.e. exactly as written down)?