Respect prefix when checking if selector selects

Prefer context functions for brevity. Avoid intermediate collections.
Check scope type
Use type test of prefix for usages in scope
Style tweaks in CheckUnused
Attachment tracks derivation
No warn serialization methods
Assume tpd

Author: som-snytt

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -3295,14 +3295,13 @@ extends Message(UnusedSymbolID) {
   override def explain(using Context) = ""
 }
 
-object UnusedSymbol {
-    def imports(using Context): UnusedSymbol = new UnusedSymbol(i"unused import")
-    def localDefs(using Context): UnusedSymbol = new UnusedSymbol(i"unused local definition")
-    def explicitParams(using Context): UnusedSymbol = new UnusedSymbol(i"unused explicit parameter")
-    def implicitParams(using Context): UnusedSymbol = new UnusedSymbol(i"unused implicit parameter")
-    def privateMembers(using Context): UnusedSymbol = new UnusedSymbol(i"unused private member")
-    def patVars(using Context): UnusedSymbol = new UnusedSymbol(i"unused pattern variable")
-}
+object UnusedSymbol:
+  def imports(using Context): UnusedSymbol = UnusedSymbol(i"unused import")
+  def localDefs(using Context): UnusedSymbol = UnusedSymbol(i"unused local definition")
+  def explicitParams(using Context): UnusedSymbol = UnusedSymbol(i"unused explicit parameter")
+  def implicitParams(using Context): UnusedSymbol = UnusedSymbol(i"unused implicit parameter")
+  def privateMembers(using Context): UnusedSymbol = UnusedSymbol(i"unused private member")
+  def patVars(using Context): UnusedSymbol = UnusedSymbol(i"unused pattern variable")
 
 class NonNamedArgumentInJavaAnnotation(using Context) extends SyntaxMsg(NonNamedArgumentInJavaAnnotationID):