Fix #7781: Avoid using nested package definition

Had to remove `sealed` from `Expr`

Author: nicolasstucki

library/src/scala/internal/quoted/TastyTreeExpr.scala

@@ -0,0 +1,22 @@
+package scala.internal.quoted
+
+import scala.quoted.Expr
+
+/** An Expr backed by a tree. Only the current compiler trees are allowed.
+  *
+  *  These expressions are used for arguments of macros. They contain and actual tree
+  *  from the program that is being expanded by the macro.
+  *
+  *  May contain references to code defined outside this TastyTreeExpr instance.
+  */
+final class TastyTreeExpr[Tree](val tree: Tree, val scopeId: Int) extends Expr[Any] {
+  override def equals(that: Any): Boolean = that match {
+    case that: TastyTreeExpr[_] =>
+      // TastyTreeExpr are wrappers around trees, therfore they are equals if their trees are equal.
+      // All scopeId should be equal unless two different runs of the compiler created the trees.
+      tree == that.tree && scopeId == that.scopeId
+    case _ => false
+  }
+  override def hashCode: Int = tree.hashCode
+  override def toString: String = s"Expr(<tasty tree>)"
+}