Prepare `...Group::Treesaver` to ndjson

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

app/services/groups/import_export/export_service.rb

@@ -56,7 +56,12 @@ module Groups
       end
 
       def tree_exporter
-        Gitlab::ImportExport::Group::TreeSaver.new(group: @group, current_user: @current_user, shared: @shared, params: @params)
+        Gitlab::ImportExport::Group::LegacyTreeSaver.new(
+          group: @group,
+          current_user: @current_user,
+          shared: @shared,
+          params: @params
+        )
       end
 
       def file_saver

ee/spec/lib/ee/gitlab/import_export/group/tree_saver_spec.rb → ee/spec/lib/ee/gitlab/import_export/group/legacy_tree_saver_spec.rb

@@ -2,7 +2,7 @@
 
 require 'spec_helper'
 
-describe Gitlab::ImportExport::Group::TreeSaver do
+describe Gitlab::ImportExport::Group::LegacyTreeSaver do
   describe 'saves the group tree into a json object' do
     let_it_be(:user) { create(:user) }
     let_it_be(:group) { create(:group) }

lib/gitlab/import_export/group/tree_saver.rb → lib/gitlab/import_export/group/legacy_tree_saver.rb

@@ -3,7 +3,7 @@
 module Gitlab
   module ImportExport
     module Group
-      class TreeSaver
+      class LegacyTreeSaver
         attr_reader :full_path, :shared
 
         def initialize(group:, current_user:, shared:, params: {})

spec/lib/gitlab/import_export/group/tree_saver_spec.rb → spec/lib/gitlab/import_export/group/legacy_tree_saver_spec.rb

@@ -2,7 +2,7 @@
 
 require 'spec_helper'
 
-describe Gitlab::ImportExport::Group::TreeSaver do
+describe Gitlab::ImportExport::Group::LegacyTreeSaver do
   describe 'saves the group tree into a json object' do
     let(:shared) { Gitlab::ImportExport::Shared.new(group) }
     let(:group_tree_saver) { described_class.new(group: group, current_user: user, shared: shared) }
@@ -28,7 +28,7 @@ describe Gitlab::ImportExport::Group::TreeSaver do
     # except:
     # context 'with description override' do
     # context 'group members' do
-    # ^ These are specific for the Group::TreeSaver
+    # ^ These are specific for the Group::LegacyTreeSaver
     context 'JSON' do
       let(:saved_group_json) do
         group_tree_saver.save

spec/services/groups/import_export/export_service_spec.rb

@@ -50,7 +50,7 @@ describe Groups::ImportExport::ExportService do
     end
 
     it 'saves the models' do
-      expect(Gitlab::ImportExport::Group::TreeSaver).to receive(:new).and_call_original
+      expect(Gitlab::ImportExport::Group::LegacyTreeSaver).to receive(:new).and_call_original
 
       service.execute
     end