[2.0.x] fix: Fixes metabuild reloading (#9019)

**Problem**
There seems to be multiple problems around metabuild reloading (reload plugins).

1. Originally reported issue 9006 states that the build can't come back
   from reload plugins.
2. During the course of fixing, I discovered that when reload plugins is used
   the project name is set to "project" instead of foo-build etc.
3. Also there was a bug in the rootPaths when reload plugins is used,
   which results in class not defined error.

**Solution**
1. Fix the plugin context so reload plugin still behaves like a metabuild.
2. Fix the rootPaths.

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

@@ -73,10 +73,10 @@ private[sbt] object Load {
     val launcher = scalaProvider.launcher
     val stagingDirectory = getStagingDirectory(state, globalBase).getCanonicalFile
     val javaHome = Util.javaHome
-    val out = baseDirectory.toPath.resolve("target").resolve("out")
+    val out = app.baseDirectory.toPath.resolve("target").resolve("out")
     val rootPaths = Map(
       "OUT" -> out,
-      "BASE" -> baseDirectory.toPath,
+      "BASE" -> app.baseDirectory.toPath,
       "SBT_BOOT" -> launcher.bootDirectory.toPath,
       "IVY_HOME" -> launcher.ivyHome.toPath,
       "JAVA_HOME" -> javaHome,
@@ -119,7 +119,9 @@ private[sbt] object Load {
     )
     val evalPluginDef: (BuildStructure, State) => PluginData = EvaluateTask.evalPluginDef
     val delegates = defaultDelegates
-    val pluginMgmt = PluginManagement(loader)
+    import sbt.ProjectExtra.projectReturn
+    val pluginContext = PluginManagement.Context(false, Project.projectReturn(state).size - 1)
+    val pluginMgmt = PluginManagement(loader, pluginContext)
     val inject = InjectSettings(injectGlobal(state), Nil, const(Nil))
     SysProp.setSwovalTempDir()
     SysProp.setIpcSocketTempDir()

main/src/main/scala/sbt/internal/PluginManagement.scala

@@ -54,12 +54,15 @@ object PluginManagement {
   val emptyContext: Context = Context(false, 0)
 
   def apply(initialLoader: ClassLoader): PluginManagement =
+    PluginManagement(initialLoader, emptyContext)
+
+  def apply(initialLoader: ClassLoader, context: Context): PluginManagement =
     PluginManagement(
       Set.empty,
       Set.empty,
       new PluginClassLoader(initialLoader),
       initialLoader,
-      emptyContext
+      context
     )
 
   def extractOverrides(classpath: Classpath): Set[ModuleID] =

sbt-app/src/sbt-test/project-load/metabuild/project/FooPlugin.scala

@@ -0,0 +1,24 @@
+package example
+
+import sbt.*
+import Keys.*
+import complete.DefaultParsers.{ *, given }
+
+object FooPlugin extends AutoPlugin:
+  override def requires = empty
+  override def trigger = allRequirements
+
+  lazy object autoImport:
+    @transient
+    lazy val foo = taskKey[Unit]("foo")
+    lazy val check = inputKey[Unit]("check")
+
+  import autoImport.*
+  override def projectSettings: Seq[Def.Setting[?]] = Seq(
+    foo := println("foo"),
+    check := {
+      val args = spaceDelimited("<arg>").parsed
+      assert(name.value.endsWith(args.head), s"${name.value} does not end with ${args.head}")
+    },
+  )
+end FooPlugin

sbt-app/src/sbt-test/project-load/metabuild/project/project/FooPlugin.scala

@@ -0,0 +1,24 @@
+package example
+
+import sbt.*
+import Keys.*
+import complete.DefaultParsers.{ *, given }
+
+object FooPlugin extends AutoPlugin:
+  override def requires = empty
+  override def trigger = allRequirements
+
+  lazy object autoImport:
+    @transient
+    lazy val foo = taskKey[Unit]("foo")
+    lazy val check = inputKey[Unit]("check")
+
+  import autoImport.*
+  override def projectSettings: Seq[Def.Setting[?]] = Seq(
+    foo := println("foo"),
+    check := {
+      val args = spaceDelimited("<arg>").parsed
+      assert(name.value.endsWith(args.head), s"${name.value} does not end with ${args.head}")
+    },
+  )
+end FooPlugin

sbt-app/src/sbt-test/project-load/metabuild/test

@@ -0,0 +1,9 @@
+> foo
+> reload plugins
+> check -build
+> foo
+> reload plugins
+> reload return
+> reload return
+> compile
+> foo