[2.x] fix: Fix scala-reflect not found problem (#8632)

## Problem
We enforce same-version policy for scala-reflect in Scala 2.13.
However due to sandwich dependency, the graph can bump
scala-library to 3.8.1, which is missing scala-reflect counterpart.

## Solution
Drop the same-version policy.

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

@@ -3134,17 +3134,16 @@ object Classpaths {
       }).value),
     csrSameVersions ++= {
       partialVersion(scalaVersion.value) match {
-        // See https://github.com/sbt/sbt/issues/8224
+        case Some((major, minor)) if major == 2 && minor < 13 =>
-        // Scala 3.8+ should align only Scala3_8Artifacts
-        case Some((3, minor)) if minor >= 8 =>
-          ScalaArtifacts.Scala3_8Artifacts
-            .map(a => InclExclRule(scalaOrganization.value, a))
-            .toSet :: Nil
-        case Some((major, minor)) if major == 2 || major == 3 =>
           ScalaArtifacts.Artifacts
             .map(a => InclExclRule(scalaOrganization.value, a))
             .toSet :: Nil
-        case _ => Nil
+        // Due to the Scala 2.13-3.x sandwich, the absence of scala-reflect
+        // that corresponds with scala-library 3.8.x propagates to 2.13 builds as well.
+        case _ =>
+          ScalaArtifacts.Scala3_8Artifacts
+            .map(a => InclExclRule(scalaOrganization.value, a))
+            .toSet :: Nil
       }
     },
     moduleName := normalizedName.value,

sbt-app/src/sbt-test/dependency-management/stdlib-3.8/build.sbt

@@ -0,0 +1,12 @@
+lazy val a = project
+  .settings(
+    scalaVersion := "3.8.1"
+  )
+
+// (b / update) sbt.librarymanagement.ResolveException: Error downloading org.scala-lang:scala-reflect:3.8.1
+lazy val b = project
+  .dependsOn(a)
+  .settings(
+    scalaVersion := "2.13.10",
+    libraryDependencies += "org.scala-lang" % "scala-reflect" % "2.13.10",
+  )

sbt-app/src/sbt-test/dependency-management/stdlib-3.8/test

@@ -0,0 +1 @@
+> update

sbt-app/src/sbt-test/dependency-management/stdlib-unfreeze/build.sbt

@@ -3,12 +3,12 @@ import sbt.librarymanagement.InclExclRule
 lazy val a = project.settings(
   scalaVersion := "2.13.11",
   libraryDependencies += "org.scala-lang" % "scala-reflect" % scalaVersion.value,
-  TaskKey[Unit]("checkLibs") := checkLibs("2.13.11", (Compile/dependencyClasspath).value, ".*scala-(library|reflect).*"),
+  TaskKey[Unit]("checkLibs") := checkLibs("2.13.11", (Compile/dependencyClasspath).value, ".*scala-library.*"),
 )
 
 lazy val b = project.dependsOn(a).settings(
   scalaVersion := "2.13.12",
-  TaskKey[Unit]("checkLibs") := checkLibs("2.13.12", (Compile/dependencyClasspath).value, ".*scala-(library|reflect).*"),
+  TaskKey[Unit]("checkLibs") := checkLibs("2.13.12", (Compile/dependencyClasspath).value, ".*scala-library.*"),
 )
 
 lazy val a3 = project.settings(

sbt-app/src/sbt-test/dependency-management/stdlib-unfreeze/test

@@ -15,6 +15,5 @@ $ delete s2.13.14.txt
 # without the default `csrSameVersions`, scala-reflect in b stays at 2.13.11
 > set b/csrSameVersions := Nil
 > b/update
--> b/checkLibs
 
 > ak/checkLibs