Stop importing TimestampedData from package:analyzer/src. (#4091)

As of analyzer version 8.1.0, the `TimestampedData` class is available
in the analyzer public API at
`package:analyzer/source/timestamped_data.dart`.

Importing `TimestampedData` via the public API will allow analyzer
internals to be cleaned up in the future.

lib/src/model/package_graph.dart

diff --git a/lib/src/model/package_graph.dart b/lib/src/model/package_graph.dart
index a329614..d6dc245 100644
--- a/lib/src/model/package_graph.dart
+++ b/lib/src/model/package_graph.dart
@@ -8,6 +8,7 @@
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/source/source.dart';
+import 'package:analyzer/source/timestamped_data.dart';
 // ignore: implementation_imports
 import 'package:analyzer/src/dart/ast/ast.dart';
 // ignore: implementation_imports
@@ -15,8 +16,6 @@
     show InheritanceManager3;
 // ignore: implementation_imports
 import 'package:analyzer/src/generated/sdk.dart' show DartSdk, SdkLibrary;
-// ignore: implementation_imports
-import 'package:analyzer/src/generated/timestamped_data.dart';
 import 'package:collection/collection.dart';
 import 'package:dartdoc/src/dartdoc_options.dart';
 import 'package:dartdoc/src/failure.dart';