[flutter_plugin_tools] Overhaul drive-examples (#4099)

Significantly restructures drive-examples:
- Migrates it to the new package-looping base command
- Enforces that only one platform is passed, since in practice multiple platforms never actually worked. (The logic is structured so that it will be easy to enable multi-platform if `flutter drive` gains multi-platform support.)
- Fixes the issue where `--ios` and `--android` were semi-broken, by doing explicit device targeting for them rather than relying on the default device being the right kind
- Extracts much of the logic to helpers so it's easier to understand the flow
- Removes support for a legacy integration test file structure that is no longer used
- Adds more test coverage; previously no failure cases were actually tested.

Fixes flutter/flutter#85147
Part of flutter/flutter#83413

Author: stuartmorgan-g

script/tool/lib/src/build_examples_command.dart

@@ -6,7 +6,6 @@ import 'dart:async';
 
 import 'package:file/file.dart';
 import 'package:path/path.dart' as p;
-import 'package:platform/platform.dart';
 
 import 'common/core.dart';
 import 'common/package_looping_command.dart';
@@ -151,8 +150,6 @@ class BuildExamplesCommand extends PackageLoopingCommand {
     String flutterBuildType, {
     List<String> extraBuildFlags = const <String>[],
   }) async {
-    final String flutterCommand =
-        const LocalPlatform().isWindows ? 'flutter.bat' : 'flutter';
     final String enableExperiment = getStringArg(kEnableExperiment);
 
     final int exitCode = await processRunner.runAndStream(

script/tool/lib/src/common/plugin_command.dart

@@ -8,6 +8,7 @@ import 'package:args/command_runner.dart';
 import 'package:file/file.dart';
 import 'package:git/git.dart';
 import 'package:path/path.dart' as p;
+import 'package:platform/platform.dart';
 
 import 'core.dart';
 import 'git_version_finder.dart';
@@ -85,6 +86,10 @@ abstract class PluginCommand extends Command<void> {
   int? _shardIndex;
   int? _shardCount;
 
+  /// The command to use when running `flutter`.
+  String get flutterCommand =>
+      const LocalPlatform().isWindows ? 'flutter.bat' : 'flutter';
+
   /// The shard of the overall command execution that this instance should run.
   int get shardIndex {
     if (_shardIndex == null) {