SBT-Plugin: expose sbt dependency versions to build

My project has many submodules which I include in my main-project.. its just a way to split things up, to keep it more organised.
In those submodules, I try to keep the external dependencies to a minimum.
Some things, like Play-Cache, Play-WS or Play-JSON I need in some modules.

So what I do (until now) is to explicitly define those dependencies in my subproject`s build, like

libraryDependencies += "com.typesafe.play" %% "play-json" % "2.6.0-M6"

Those dependency versions are all stored in
https://github.com/playframework/playframework/blob/c81dbfaa4a51da8b42b69bc12efe9a63f6868553/framework/project/Dependencies.scala

However, this file is not available for me in my build.sbt file, as its not part of the sbt-plugin.
The only thing I currently have available is
https://github.com/playframework/playframework/blob/c81dbfaa4a51da8b42b69bc12efe9a63f6868553/framework/src/sbt-plugin/src/main/scala/play/sbt/PlayImport.scala (and the other classes in that package)

So, at the moment, I always have to manually adjust my various dependency versions when changing the version of the Play Plugin.
Ideally, those dependency versions would also be exposed through the Play-SBT-Plugin, so that I can refer to them in my build.

As for the example of Play-JSON: Before I could just do a
libraryDependencies += PlayImport.json .. now that its "movedExternal" I cannot refer to it.. also the version is not available for me anymore.

Start by comparing framework/project/Dependencies.scala with framework/src/sbt-plugin/src/main/scala/play/sbt/PlayImport.scala, focusing on how dependency versions and movedExternal entries are represented. Trace what is currently exposed to build.sbt and define the public references needed for external projects. Done means a subproject can use the Play SBT plugin to refer to these dependency versions without duplicating them.