!str #15977 Rename and change types in the new DSL

* Cleaned up type signatures
* Changed names of Source/Sink/ProcessorFlow to Tap/Drain/Pipe
* Introduced new Source/Sink/Flow
* Made all Pipe types package private
* Changed the TypeGraph DSL to work on Flow/Sink/Source
Merge pull request #16007 from akka/wip-15977-changin-all-the-small-dsl-things-ban

+str #15977 Rename and change types in the new DSL