Make Configuration's copy private[this]

.. now that it doesn't need to share it with a parent type.

build.sbt

@@ -110,6 +110,10 @@ lazy val lmCore = (project in file("core"))
       exclude[MissingClassProblem]("sbt.internal.librarymanagement.InlineConfigurationFunctions"),
       // dropped internal class parent (InlineConfigurationFunctions)
       exclude[MissingTypesProblem]("sbt.librarymanagement.ModuleDescriptorConfiguration$"),
+
+      // Configuration's copy method was never meant to be public
+      exclude[DirectMissingMethodProblem]("sbt.librarymanagement.Configuration.copy"),
+      exclude[DirectMissingMethodProblem]("sbt.librarymanagement.Configuration.copy$default$*"),
     ),
   )
   .configure(addSbtIO, addSbtUtilLogging, addSbtUtilPosition, addSbtUtilCache)

core/src/main/scala/sbt/librarymanagement/Configuration.scala

@@ -35,7 +35,7 @@ final class Configuration private[sbt] (
 
   override def toString: String = name
 
-  protected[this] def copy(
+  private[this] def copy(
       id: String = id,
       name: String = name,
       description: String = description,