Fixes #2415. Hide global setting fix behind sbt.globalsettingfix

We are hiding a bug fix on global setting that was not importing auto
imports because fixing this via sbt/sbt#2399 breaks the source
compatibility: sbt/sbt#2415
I've split out the relevant portion of the scripted test into
global-settings, and marked it pending.

main/src/main/scala/sbt/Load.scala

@@ -71,12 +71,19 @@ object Load {
         if (files.isEmpty || base == globalBase) const(Nil) else buildGlobalSettings(globalBase, files, config)
       config.copy(injectSettings = config.injectSettings.copy(projectLoaded = compiled))
     }
+  // We are hiding a bug fix on global setting that was not importing auto imports.
+  // Because fixing this via https://github.com/sbt/sbt/pull/2399
+  // breaks the source compatibility: https://github.com/sbt/sbt/issues/2415
+  @deprecated("Remove this when we can break source compatibility.", "0.13.10")
+  private[sbt] def useAutoImportInGlobal = sys.props.get("sbt.globalsettingfix") map { _.toLowerCase == "true" } getOrElse false
   def buildGlobalSettings(base: File, files: Seq[File], config: sbt.LoadBuildConfiguration): ClassLoader => Seq[Setting[_]] =
     {
       val eval = mkEval(data(config.globalPluginClasspath), base, defaultEvalOptions)
 
       val imports = BuildUtil.baseImports ++
-        config.detectedGlobalPlugins.imports
+        (// when we can beak the source compat, remove this if and use config.detectedGlobalPlugins.imports
+        if (useAutoImportInGlobal) config.detectedGlobalPlugins.imports
+        else BuildUtil.importAllRoot(config.globalPluginNames))
 
       loader => {
         val loaded = EvaluateConfigurations(eval, files, imports)(loader)

sbt/src/sbt-test/project/global-plugin/global/plugins/A.scala

@@ -1,15 +1,5 @@
 package test
 
-import sbt._
-
 object Global {
   val x = 3
 }
-
-object GlobalAutoPlugin extends AutoPlugin {
-
-  object autoImport {
-    lazy val globalAutoPluginSetting = settingKey[String]("A top level setting declared in a plugin.")
-  }
-
-}

sbt/src/sbt-test/project/global-settings/global/plugins/A.scala

@@ -0,0 +1,11 @@
+package test
+
+import sbt._
+
+object GlobalAutoPlugin extends AutoPlugin {
+
+  object autoImport {
+    lazy val globalAutoPluginSetting = settingKey[String]("A top level setting declared in a plugin.")
+  }
+
+}

sbt/src/sbt-test/project/global-settings/pending

@@ -0,0 +1,3 @@
+# This test is marked pending because sbt.globalsettingfix flag is off by default
+# See https://github.com/sbt/sbt/issues/2415
+> name