Add sources of synthetic classes to sources jar (#20904)

Closes #20073

Author: WojciechMazur

Diff for: library-aux/src/scala/AnyKind.scala

@@ -0,0 +1,7 @@
+package scala
+
+/** The super-type of all types.
+ *
+ *  See [[https://docs.scala-lang.org/scala3/reference/other-new-features/kind-polymorphism.html]].
+ */
+final abstract class AnyKind

Diff for: library-aux/src/scala/Matchable.scala

@@ -0,0 +1,7 @@
+package scala
+
+/** The base trait of types that can be safely pattern matched against.
+ *
+ *  See [[https://docs.scala-lang.org/scala3/reference/other-new-features/matchable.html]].
+ */
+trait Matchable

Diff for: library-aux/src/scala/andType.scala

@@ -0,0 +1,7 @@
+package scala
+
+/** The intersection of two types.
+ *
+ *  See [[https://docs.scala-lang.org/scala3/reference/new-types/intersection-types.html]].
+ */
+type &[A, B]

Diff for: library-aux/src/scala/orType.scala

@@ -0,0 +1,7 @@
+package scala
+
+/** The union of two types.
+ *
+ *  See [[https://docs.scala-lang.org/scala3/reference/new-types/union-types.html]].
+ */
+type |[A, B]

Diff for: project/Build.scala

@@ -985,7 +985,11 @@ object Build {
       "-sourcepath", (Compile / sourceDirectories).value.map(_.getAbsolutePath).distinct.mkString(File.pathSeparator),
       "-Yexplicit-nulls",
     ),
-    (Compile / doc / scalacOptions) ++= ScaladocConfigs.DefaultGenerationSettings.value.settings
+    (Compile / doc / scalacOptions) ++= ScaladocConfigs.DefaultGenerationSettings.value.settings,
+    (Compile / packageSrc / mappings) ++= {
+      val auxBase = (ThisBuild / baseDirectory).value / "library-aux/src"
+      auxBase ** "*.scala" pair io.Path.relativeTo(auxBase)
+    },
   )
 
   lazy val `scala3-library` = project.in(file("library")).asDottyLibrary(NonBootstrapped)