In #7096 we started a transition to valid Maven patterns of sbt plugins. The code for handling backward and forward compatible resolution is quite hacky. We should clean that code in sbt 2.x and publish sbt plugins using a cross version prefix (_sbt2 or _sbt1.0) as we do for other platforms, such as Scala.js or Scala Native.

As consequences:

• sbt 2.x won't be able to resolve plugins published with sbt 1.8-
• sbt 1.8- won't be able to resolve plugins published with sbt 2.x