[2.x] refactor: Gate Ivy-specific tasks behind useIvy

**Problem**
Several tasks (makePom, makeMavenPomOfSbtPlugin, deliverTask, publisher,
publishOrSkip, depMap, GlobalPlugin dependency mapping) unconditionally
depend on Ivy infrastructure, preventing ivyless operation.

**Solution**
Add useIvy guards to each task:
- makePom and makeMavenPomOfSbtPlugin: error when useIvy=false
- deliverTask: error when useIvy=false
- publisher: error when useIvy=false
- publishOrSkip: error when useIvy=false
- depMap: return empty map when useIvy=false
- GlobalPlugin.extract: skip ivyModule.dependencyMapping when useIvy=false

Generated-by: Claude claude-opus-4-6

main/src/main/scala/sbt/Defaults.scala

@@ -3724,6 +3724,7 @@ object Classpaths {
   def deliverTask(config: TaskKey[PublishConfiguration]): Initialize[Task[File]] =
     Def.task {
       Def.unit(update.value)
+      if !useIvy.value then sys.error("deliver/makeIvyXml requires useIvy := true")
       IvyActions.deliver(ivyModule.value, config.value, streams.value.log)
     }
 
@@ -3755,6 +3756,10 @@ object Classpaths {
           val log = streams.value.log
           val ref = thisProjectRef.value
           logSkipPublish(log, ref)
+        } else if (!useIvy.value) {
+          sys.error(
+            "publishOrSkip requires useIvy := true. Use publish/publishLocal for ivyless publishing."
+          )
         } else {
           val conf = config.value
           val log = streams.value.log
@@ -4212,10 +4217,10 @@ object Classpaths {
 
   private[sbt] def depMap: Initialize[Task[Map[ModuleRevisionId, ModuleDescriptor]]] =
     import sbt.TupleSyntax.*
-    (buildDependencies.toTaskable, thisProjectRef.toTaskable, settingsData, streams).flatMapN {
-      (bd, thisProj, data, s) =>
+    (buildDependencies.toTaskable, thisProjectRef.toTaskable, settingsData, streams)
+      .flatMapN { (bd, thisProj, data, s) =>
         depMap(bd.classpathTransitiveRefs(thisProj), data, s.log)
-    }
+      }
 
   private[sbt] def depMap(
       projects: Seq[ProjectRef],

main/src/main/scala/sbt/internal/GlobalPlugin.scala

@@ -79,7 +79,9 @@ object GlobalPlugin {
     val taskInit = Def.task {
       val intcp = (Runtime / internalDependencyClasspath).value
       val prods = (Runtime / exportedProducts).value
-      val depMap = projectDescriptors.value + ivyModule.value.dependencyMapping(state.log)
+      val depMap =
+        if useIvy.value then projectDescriptors.value + ivyModule.value.dependencyMapping(state.log)
+        else projectDescriptors.value
 
       GlobalPluginData(
         projectID.value,