feat: plugin cross building as Scala cross building

**Problem**
Since Scala cross building works better than the plugin cross building `^^`,
it was common for plugin authors to encode plugin cross building as Scala cross building
given that we usually have zero or one sbt release in one Scala version.

**Solution**
This brings in the setting into SbtPlugin so plugin authors can cross build
using sbt 2.x.

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

@@ -332,9 +332,6 @@ object Defaults extends BuildCommon {
       envVars :== Map.empty,
       sbtVersion := appConfiguration.value.provider.id.version,
       sbtBinaryVersion := binarySbtVersion(sbtVersion.value),
-      // `pluginCrossBuild` scoping is based on sbt-cross-building plugin.
-      // The idea here is to be able to define a `sbtVersion in pluginCrossBuild`, which
-      // directs the dependencies of the plugin to build to the specified sbt plugin version.
       pluginCrossBuild / sbtVersion := sbtVersion.value,
       onLoad := idFun[State],
       onUnload := idFun[State],
@@ -3102,7 +3099,7 @@ object Classpaths {
     Defaults.globalDefaults(
       Seq(
         publishMavenStyle :== true,
-        sbtPluginPublishLegacyMavenStyle :== true,
+        sbtPluginPublishLegacyMavenStyle :== false,
         publishArtifact :== true,
         (Test / publishArtifact) :== false
       )

main/src/main/scala/sbt/plugins/SbtPlugin.scala

@@ -10,12 +10,19 @@ package sbt
 package plugins
 
 import sbt.Def.Setting
-import sbt.Keys._
+import sbt.Keys.*
+import sbt.SlashSyntax0.*
 
-object SbtPlugin extends AutoPlugin {
+object SbtPlugin extends AutoPlugin:
   override def requires = ScriptedPlugin
 
   override lazy val projectSettings: Seq[Setting[_]] = Seq(
-    sbtPlugin := true
+    sbtPlugin := true,
+    pluginCrossBuild / sbtVersion := {
+      scalaBinaryVersion.value match
+        case "3"    => sbtVersion.value
+        case "2.12" => "1.5.8"
+        case "2.10" => "0.13.18"
+    },
   )
-}
+end SbtPlugin

sbt-app/src/sbt-test/project-matrix/plugin/build.sbt

@@ -0,0 +1,28 @@
+val scala3 = "3.3.3"
+val scala212 = "2.12.19"
+
+organization := "com.example"
+version := "0.1.0-SNAPSHOT"
+
+lazy val checkSbt2Plugin = taskKey[Unit]("")
+
+lazy val plugin = (projectMatrix in file("plugin"))
+  .enablePlugins(SbtPlugin)
+  .settings(
+    organization := "com.example",
+    name := "sbt-example",
+    // TODO: Once 2.0 is released we can move this check to the `test` file
+    checkSbt2Plugin := {
+      val repo = (ThisBuild / baseDirectory).value / "repo"
+      val sbtV = sbtBinaryVersion.value
+      val expected = (repo / "com" / "example" /
+        s"sbt-example_sbt${sbtV}_3" /
+        "0.1.0-SNAPSHOT" /
+        s"sbt-example_sbt${sbtV}_3-0.1.0-SNAPSHOT.pom")
+      assert(expected.exists, s"$expected did not exist")
+    },
+  )
+  .jvmPlatform(scalaVersions = Seq(scala3, scala212))
+
+publishMavenStyle := true
+publishTo := Some(Resolver.file("test-publish", (ThisBuild / baseDirectory).value / "repo/"))

sbt-app/src/sbt-test/project-matrix/plugin/test

@@ -0,0 +1,8 @@
+> compile
+
+> publish
+
+$ exists repo/com/example/sbt-example_2.12_1.0/0.1.0-SNAPSHOT/sbt-example_2.12_1.0-0.1.0-SNAPSHOT.pom
+$ absent repo/com/example/sbt-example_2.12_1.0/0.1.0-SNAPSHOT/sbt-example-0.1.0-SNAPSHOT.pom
+
+> checkSbt2Plugin