[tool/ci] Add iOS/macOS and Dart support to fetch-deps (#4562)

Adds `fetch-deps` support for:
- iOS/macOS dependencies, using `pod install`
- Dart package dependencies, using `pub get`

To make avoid doing extra work in the Dart dependencies step when using this with `*_platform_tests` CI, also adds flags for all of the other platforms, and adds a flag that allows skipping Dart dependencies for any package that doesn't have an example supporting any requested platform. This means that we can pass, e.g., `--windows --supporting-target-platforms-only` to only fetch Dart packages for packages with examples that will be build during the build-and-drive Windows tests.

Adds this as a new step in every platform tests CI task, and in the standard analyze step, so that we will pre-fetch Dart packages (and for iOS/macOS, pods). This won't yet fully eliminate later network access (see https://github.com/flutter/flutter/issues/131204), but will give us early warning on any major failures, such as pub being entirely unreachable from the bots.
- These are marked as an infrastructure step; we'll have to see if this ends up being confusing in practice. If `pub` resolution fails for legitimate reasons, such as a PR that tries to require a version of a package that doesn't exist or that has conflicts, this will cause a failure that is marked as infra. My assumption is that the much more common case is going to be that it is actually an infra failure.

Fixes https://github.com/flutter/flutter/issues/130280

script/tool/lib/src/build_examples_command.dart

@@ -176,9 +176,8 @@ class BuildExamplesCommand extends PackageLoopingCommand {
         // supported platforms. For packages, just log and skip any requested
         // platform that a package doesn't have set up.
         if (!isPlugin &&
-            !example.directory
-                .childDirectory(platform.flutterPlatformDirectory)
-                .existsSync()) {
+            !example.appSupportsPlatform(
+                getPlatformByName(platform.pluginPlatform))) {
           print('Skipping ${platform.label} for $packageName; not supported.');
           continue;
         }
@@ -304,11 +303,6 @@ class _PlatformDetails {
   /// The `flutter build` build type.
   final String flutterBuildType;
 
-  /// The Flutter platform directory name.
-  // In practice, this is the same as the plugin platform key for all platforms.
-  // If that changes, this can be adjusted.
-  String get flutterPlatformDirectory => pluginPlatform;
-
   /// Any extra flags to pass to `flutter build`.
   final List<String> extraBuildFlags;
 }