Restore binary compatibility for LogManager.defaults

Although log manager is in the internal package, akka uses it and it
seems worth preserving binary compatibility in their build, at least for
sbt 1.4.0-M2. Once the community build is passing, we can consider
reverting this.

main/src/main/scala/sbt/Keys.scala

@@ -27,7 +27,7 @@ import sbt.internal.io.WatchState
 import sbt.internal.librarymanagement.{ CompatibilityWarningOptions, IvySbt }
 import sbt.internal.remotecache.RemoteCacheArtifact
 import sbt.internal.server.ServerHandler
-import sbt.internal.util.{ Appender, AttributeKey, ProgressState, SourcePosition }
+import sbt.internal.util.{ AttributeKey, ProgressState, SourcePosition }
 import sbt.io._
 import sbt.librarymanagement.Configurations.CompilerPlugin
 import sbt.librarymanagement.LibraryManagementCodec._
@@ -61,7 +61,7 @@ object Keys {
   val timingFormat = settingKey[java.text.DateFormat]("The format used for displaying the completion time.").withRank(CSetting)
   @deprecated("", "1.4.0")
   val extraLoggers = settingKey[ScopedKey[_] => Seq[XAppender]]("A function that provides additional loggers for a given setting.").withRank(DSetting)
-  val extraAppenders = settingKey[ScopedKey[_] => Seq[Appender]]("A function that provides additional loggers for a given setting.").withRank(DSetting)
+  val extraAppenders = settingKey[AppenderSupplier]("A function that provides additional loggers for a given setting.").withRank(DSetting)
   val useLog4J = settingKey[Boolean]("Toggles whether or not to use log4j for sbt internal loggers.").withRank(Invisible)
   val logManager = settingKey[LogManager]("The log manager, which creates Loggers for different contexts.").withRank(DSetting)
   private[sbt] val loggerContext = AttributeKey[LoggerContext]("sbt-logger-context", "The logger config which creates Loggers for different contexts.", Int.MaxValue)

main/src/main/scala/sbt/internal/LogManager.scala

@@ -16,6 +16,7 @@ import sbt.Scope.GlobalScope
 import sbt.internal.util.MainAppender._
 import sbt.internal.util._
 import sbt.util.{ Level, LogExchange, Logger, LoggerContext }
+import org.apache.logging.log4j.core.{ Appender => XAppender }
 
 sealed abstract class LogManager {
   def apply(
@@ -44,6 +45,14 @@ sealed abstract class LogManager {
     backgroundLog(data, state, task, LoggerContext.globalContext)
 }
 
+/**
+ * A functional interface that allows us to preserve binary compatibility
+ * for LogManager.defaults with the old log4j variant.
+ */
+trait AppenderSupplier {
+  def apply(s: ScopedKey[_]): Seq[Appender]
+}
+
 object LogManager {
   import java.util.concurrent.atomic.AtomicInteger
   private val generateId: AtomicInteger = new AtomicInteger
@@ -75,8 +84,14 @@ object LogManager {
   def defaultManager(console: ConsoleOut): LogManager =
     withLoggers((_, _) => defaultScreen(console))
 
+  @deprecated(
+    "use defaults that takes AppenderSupplier instead of ScopedKey[_] => Seq[Appender]",
+    "1.4.0"
+  )
+  def defaults(extra: ScopedKey[_] => Seq[XAppender], console: ConsoleOut): LogManager =
+    defaults((sk: ScopedKey[_]) => extra(sk).map(new ConsoleAppenderFromLog4J("extra", _)), console)
   // This is called by Defaults.
-  def defaults(extra: ScopedKey[_] => Seq[Appender], console: ConsoleOut): LogManager =
+  def defaults(extra: AppenderSupplier, console: ConsoleOut): LogManager =
     withLoggers(
       (task, state) => defaultScreen(console, suppressedMessage(task, state)),
       extra = extra
@@ -89,14 +104,14 @@ object LogManager {
       screen: (ScopedKey[_], State) => Appender = (_, s) => defaultScreen(s.globalLogging.console),
       backed: PrintWriter => Appender = defaultBacked,
       relay: Unit => Appender = defaultRelay,
-      extra: ScopedKey[_] => Seq[Appender] = _ => Nil
+      extra: AppenderSupplier = _ => Nil
   ): LogManager = new DefaultLogManager(screen, backed, relay, extra)
 
   private class DefaultLogManager(
       screen: (ScopedKey[_], State) => Appender,
       backed: PrintWriter => Appender,
       relay: Unit => Appender,
-      extra: ScopedKey[_] => Seq[Appender]
+      extra: AppenderSupplier
   ) extends LogManager {
     def apply(
         data: Settings[Scope],