add scala3 library to tasty-core-scala2 project

Author: bishabosha

project/Build.scala

@@ -2094,7 +2094,16 @@ object Build {
         }
       )
 
-    def asTastyCoreScala2: Project = project.settings(commonScala2Settings)
+    def asTastyCoreScala2: Project = project
+      .settings(commonScala2Settings)
+      // need to add @annotation.internal.sharable to the classpath for compiling
+      // we don't actually publish this library anywhere, so it's fine.
+      // if someone depends on the sources of tasty-core in a scala 2 project,
+      // they should strip the sharable annotation, or add -Ytasty-reader
+      .dependsOn(dottyLibrary(NonBootstrapped) % Provided)
+      .settings(
+        scalacOptions += "-Ytasty-reader" // to read scala3 library
+      )
 
     def asDottyBench(implicit mode: Mode): Project = project.withCommonSettings.
       dependsOn(dottyCompiler).