Refine levels computation

An import now has the same precedence as a current import and definition only if
both contexts have the same owner. Imports nested in inner owners do shadow definitions
and imports in outer ones.

Author: odersky

compiler/src/dotty/tools/dotc/typer/Implicits.scala

@@ -307,7 +307,7 @@ object Implicits:
      */
     override val level: Int =
       if outerImplicits == null then 1
-      else if isImport
+      else if isImport && (irefCtx.owner eq outerImplicits.irefCtx.owner)
               || migrateTo3(using irefCtx)
               || (irefCtx.owner eq outerImplicits.irefCtx.owner)
                  && (irefCtx.scope eq outerImplicits.irefCtx.scope)