Allow imports nested in packagings to shadow

[Cherry-picked af0283f]

Author: dwijnand

compiler/src/dotty/tools/dotc/core/ContextOps.scala

@@ -135,7 +135,7 @@ object ContextOps:
     }
 
     def packageContext(tree: untpd.PackageDef, pkg: Symbol): Context = inContext(ctx) {
-      if (pkg.is(Package)) ctx.fresh.setOwner(pkg.moduleClass).setTree(tree)
+      if (pkg.is(Package)) ctx.fresh.setOwner(pkg.moduleClass).setTree(tree).setNewScope
       else ctx
     }
 end ContextOps

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

@@ -387,7 +387,7 @@ class Namer { typer: Typer =>
     def recur(stat: Tree): Context = stat match {
       case pcl: PackageDef =>
         val pkg = createPackageSymbol(pcl.pid)
-        index(pcl.stats)(using ctx.fresh.setOwner(pkg.moduleClass))
+        index(pcl.stats)(using ctx.packageContext(pcl, pkg))
         invalidateCompanions(pkg, Trees.flatten(pcl.stats map expanded))
         setDocstring(pkg, stat)
         ctx

tests/pos/i21405.scala

@@ -0,0 +1,7 @@
+package o { class IO }
+package p { class IO }
+import o._
+package q {
+  import p._
+  class D extends IO
+}