=htc Fix flow transformer naming error
=htc move source files into directories for new package structure
!htc move akka.http.{client,server,parsing,rendering} underneath akka.http.engine package
!htp rename akka.http.routing package to akka.http.server
!htk move RequestBuilding trait from akka.http.testkit into akka.http.clien package, closes #15801
Merge pull request #15936 from spray/wip-15801-mathias

Restructure HTTP package structure, move `RequestBuilding` trait from testkit into `http` module