Remove deprecated plugins (#4580)

This removes all of the deprecated plugins from the tree, as well as all references to them.

They were being left in the tree only in case they needed critical fixes before the end of this year, when they will become completely unsupported and be officially discontinued on pub.dev. The end of the year is now close enough that such a release is extremely unlikely, and they are causing some maintenance burden (e.g., the tree is currently closed on an out-of-band failure in android_alarm_manager). In the event that we do have to push an emergency fix in the next several weeks, we can either temporarily restore the plugin from git history, or use a branch.

Minor related cleanup:
- Scanning the repository for remaining references turned up that `image_picker_for_web` and `video_player_for_web` had copypasta'd the package name of its example application from one of the deprecated plugins, so this fixes those names.
- Fixes `federation-safety-check` handling of top-level files in unfederated plugins that accidentally tripped federated plugin heuristics.
- Fixes `federation-safety-check` handling of deleted plugins, as it was crashing, and tests that.
- Fixes `make-deps-path-based` handling of deleted plugins, as it was crashing, and tests that.

script/tool/lib/src/make_deps_path_based_command.dart

@@ -196,19 +196,27 @@ dependency_overrides:
   Future<Set<String>> _getNonBreakingUpdatePackages() async {
     final GitVersionFinder gitVersionFinder = await retrieveVersionFinder();
     final String baseSha = await gitVersionFinder.getBaseSha();
-    print('Finding changed packages relative to "$baseSha"\n');
+    print('Finding changed packages relative to "$baseSha"...');
 
     final Set<String> changedPackages = <String>{};
-    for (final String path in await gitVersionFinder.getChangedFiles()) {
+    for (final String changedPath in await gitVersionFinder.getChangedFiles()) {
       // Git output always uses Posix paths.
-      final List<String> allComponents = p.posix.split(path);
+      final List<String> allComponents = p.posix.split(changedPath);
       // Only pubspec changes are potential publishing events.
       if (allComponents.last != 'pubspec.yaml' ||
           allComponents.contains('example')) {
         continue;
       }
       final RepositoryPackage package =
-          RepositoryPackage(packagesDir.fileSystem.file(path).parent);
+          RepositoryPackage(packagesDir.fileSystem.file(changedPath).parent);
+      // Ignored deleted packages, as they won't be published.
+      if (!package.pubspecFile.existsSync()) {
+        final String directoryName = p.posix.joinAll(path.split(path.relative(
+            package.directory.absolute.path,
+            from: packagesDir.path)));
+        print('  Skipping $directoryName; deleted.');
+        continue;
+      }
       final String packageName = package.parsePubspec().name;
       if (!await _hasNonBreakingVersionChange(package)) {
         // Log packages that had pubspec changes but weren't included for ease