add isMetaBuild setting

Fixes #3436

This implements isMetaBuild setting that is explicitly for meta build only,
unlike sbtPlugin setting which can be used for both meta build and plugin development purpose.

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

@@ -172,6 +172,7 @@ object Defaults extends BuildCommon {
       testForkedParallel :== false,
       javaOptions :== Nil,
       sbtPlugin :== false,
+      isMetaBuild :== false,
       crossPaths :== true,
       sourcePositionMappers :== Nil,
       artifactClassifier in packageSrc :== Some(SourceClassifier),

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

@@ -234,6 +234,7 @@ object Keys {
   val classpathOptions = settingKey[ClasspathOptions]("Configures handling of Scala classpaths.").withRank(DSetting)
   val discoveredSbtPlugins = taskKey[PluginDiscovery.DiscoveredNames]("The names of sbt plugin-related modules (modules that extend Build, Plugin, AutoPlugin) defined by this project.").withRank(CTask)
   val sbtPlugin = settingKey[Boolean]("If true, enables adding sbt as a dependency and auto-generation of the plugin descriptor file.").withRank(BMinusSetting)
+  val isMetaBuild = settingKey[Boolean]("If true, this is a metabuild, a project inside project/ directory, and sbt is added to the classpath.").withRank(DSetting)
   val pluginCrossBuild = taskKey[Unit]("Dummy task to scope `sbtVersion in pluginCrossBuild`, which gets used for plugin compilation.")
   val crossSbtVersions = settingKey[Seq[String]]("The versions of Sbt used when cross-building an sbt plugin.")
   val printWarnings = taskKey[Unit]("Shows warnings from compilation, including ones that weren't printed initially.").withRank(BPlusTask)

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

@@ -22,6 +22,7 @@ import Keys.{
   exportedProducts,
   fullClasspath,
   fullResolvers,
+  isMetaBuild,
   loadedBuild,
   onLoadMessage,
   pluginData,
@@ -1180,6 +1181,7 @@ private[sbt] object Load {
   val autoPluginSettings: Seq[Setting[_]] = inScope(GlobalScope in LocalRootProject)(
     Seq(
       sbtPlugin :== true,
+      isMetaBuild :== true,
       pluginData := {
         val prod = (exportedProducts in Configurations.Runtime).value
         val cp = (fullClasspath in Configurations.Runtime).value