Prepare `...Group::TreeRestorer` to ndjson

Renaming `Gitlab::ImportExport::Group::TreeRestorer` to
`Gitlab::ImportExport::Group::LegacyTreeRestorer` is part of the effort
to enable `ndjson` on the Group Import/Export feature.
With `ndjson` enabled all the structure of restoring a group-tree is
different, so to make the maintenance easier the "non-ndjson"
structure is being renamed.

app/services/groups/import_export/import_service.rb

@@ -33,10 +33,12 @@ module Groups
       end
 
       def restorer
-        @restorer ||= Gitlab::ImportExport::Group::TreeRestorer.new(user: @current_user,
-                                                                  shared: @shared,
-                                                                  group: @group,
-                                                                  group_hash: nil)
+        @restorer ||= Gitlab::ImportExport::Group::LegacyTreeRestorer.new(
+          user: @current_user,
+          shared: @shared,
+          group: @group,
+          group_hash: nil
+        )
       end
 
       def remove_import_file

ee/spec/lib/ee/gitlab/import_export/group/tree_restorer_spec.rb → ee/spec/lib/ee/gitlab/import_export/group/legacy_tree_restorer_spec.rb

@@ -2,7 +2,7 @@
 
 require 'spec_helper'
 
-describe Gitlab::ImportExport::Group::TreeRestorer do
+describe Gitlab::ImportExport::Group::LegacyTreeRestorer do
   include ImportExport::CommonUtil
 
   let(:user) { create(:user) }

lib/gitlab/import_export/group/tree_restorer.rb → lib/gitlab/import_export/group/legacy_tree_restorer.rb

@@ -3,7 +3,7 @@
 module Gitlab
   module ImportExport
     module Group
-      class TreeRestorer
+      class LegacyTreeRestorer
         include Gitlab::Utils::StrongMemoize
 
         attr_reader :user

spec/lib/gitlab/import_export/group/tree_restorer_spec.rb → spec/lib/gitlab/import_export/group/legacy_tree_restorer_spec.rb

@@ -2,7 +2,7 @@
 
 require 'spec_helper'
 
-describe Gitlab::ImportExport::Group::TreeRestorer do
+describe Gitlab::ImportExport::Group::LegacyTreeRestorer do
   include ImportExport::CommonUtil
 
   let(:shared) { Gitlab::ImportExport::Shared.new(group) }