Small cleanup around generators (#4096)

Nits following https://github.com/dart-lang/dartdoc/pull/4095/

lib/src/dartdoc.dart

diff --git a/lib/src/dartdoc.dart b/lib/src/dartdoc.dart
index 8c847ab..26b952e 100644
--- a/lib/src/dartdoc.dart
+++ b/lib/src/dartdoc.dart
@@ -10,7 +10,6 @@
 import 'package:dartdoc/src/dartdoc_options.dart';
 import 'package:dartdoc/src/failure.dart';
 import 'package:dartdoc/src/generator/generator.dart';
-import 'package:dartdoc/src/generator/html_generator.dart';
 import 'package:dartdoc/src/logging.dart';
 import 'package:dartdoc/src/model/model.dart';
 import 'package:dartdoc/src/package_meta.dart';

lib/src/generator/generator.dart

diff --git a/lib/src/generator/generator.dart b/lib/src/generator/generator.dart
index 896e077..8e81fd6 100644
--- a/lib/src/generator/generator.dart
+++ b/lib/src/generator/generator.dart
@@ -7,6 +7,8 @@
 
 import 'package:dartdoc/src/dartdoc_options.dart';
 import 'package:dartdoc/src/generator/generator_backend.dart';
+import 'package:dartdoc/src/generator/html_generator_backend.dart';
+import 'package:dartdoc/src/generator/templates.dart';
 import 'package:dartdoc/src/logging.dart';
 import 'package:dartdoc/src/model/model.dart';
 import 'package:dartdoc/src/model_utils.dart';
@@ -306,3 +308,15 @@
             'SEO; learn more at https://goo.gl/gktN6F.'),
   ];
 }
+
+/// Creates a [Generator] with an [HtmlGeneratorBackend] backend.
+Generator initHtmlGenerator(
+  DartdocGeneratorOptionContext context, {
+  required FileWriter writer,
+}) {
+  var templates = HtmlAotTemplates();
+  var options = DartdocGeneratorBackendOptions.fromContext(context);
+  var generatorBackend = HtmlGeneratorBackend(
+      options, templates, writer, context.resourceProvider);
+  return Generator(generatorBackend);
+}

lib/src/generator/html_generator_backend.dart

diff --git a/lib/src/generator/html_generator.dart b/lib/src/generator/html_generator_backend.dart
similarity index 90%
rename from lib/src/generator/html_generator.dart
rename to lib/src/generator/html_generator_backend.dart
index 9b5c136..f64ffe1 100644
--- a/lib/src/generator/html_generator.dart
+++ b/lib/src/generator/html_generator_backend.dart
@@ -2,28 +2,14 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'package:dartdoc/src/dartdoc_options.dart';
 import 'package:dartdoc/src/generator/generator.dart';
 import 'package:dartdoc/src/generator/generator_backend.dart';
 import 'package:dartdoc/src/generator/html_resources.g.dart' as resources;
 import 'package:dartdoc/src/generator/resource_loader.dart';
 import 'package:dartdoc/src/generator/template_data.dart';
-import 'package:dartdoc/src/generator/templates.dart';
 import 'package:dartdoc/src/model/model.dart';
 import 'package:dartdoc/src/runtime_stats.dart';
 
-/// Creates a [Generator] with an [HtmlGeneratorBackend] backend.
-Generator initHtmlGenerator(
-  DartdocGeneratorOptionContext context, {
-  required FileWriter writer,
-}) {
-  var templates = HtmlAotTemplates();
-  var options = DartdocGeneratorBackendOptions.fromContext(context);
-  var generatorBackend = HtmlGeneratorBackend(
-      options, templates, writer, context.resourceProvider);
-  return Generator(generatorBackend);
-}
-
 /// Generator backend for HTML output.
 class HtmlGeneratorBackend extends GeneratorBackend {
   HtmlGeneratorBackend(

test/html_generator_test.dart

diff --git a/test/html_generator_test.dart b/test/html_generator_test.dart
index 8344af4..2873314 100644
--- a/test/html_generator_test.dart
+++ b/test/html_generator_test.dart
@@ -9,7 +9,7 @@
 import 'package:dartdoc/src/dartdoc_options.dart';
 import 'package:dartdoc/src/generator/generator.dart';
 import 'package:dartdoc/src/generator/generator_backend.dart';
-import 'package:dartdoc/src/generator/html_generator.dart';
+import 'package:dartdoc/src/generator/html_generator_backend.dart';
 import 'package:dartdoc/src/generator/html_resources.g.dart';
 import 'package:dartdoc/src/generator/templates.dart';
 import 'package:dartdoc/src/model/library.dart';