[flutter_plugin_tools] Simplify filesystem usage (#4014)

- Replaces most explicit use of `fileSystem` with path construction using the `child*` utility methods
- Removes explicit passing of a filesystem to the commands; we're already passing a `Directory` for the
  root where the tool operates, and we should never be using a different filesystem than that directory's
  filesystem, so passing it was both redundant, and a potential source of test bugs.

script/tool/lib/src/format_command.dart

@@ -20,10 +20,9 @@ final Uri _googleFormatterUrl = Uri.https('github.com',
 class FormatCommand extends PluginCommand {
   /// Creates an instance of the format command.
   FormatCommand(
-    Directory packagesDir,
-    FileSystem fileSystem, {
+    Directory packagesDir, {
     ProcessRunner processRunner = const ProcessRunner(),
-  }) : super(packagesDir, fileSystem, processRunner: processRunner) {
+  }) : super(packagesDir, processRunner: processRunner) {
     argParser.addFlag('fail-on-change', hide: true);
     argParser.addOption('clang-format',
         defaultsTo: 'clang-format',
@@ -144,7 +143,8 @@ class FormatCommand extends PluginCommand {
     final String javaFormatterPath = p.join(
         p.dirname(p.fromUri(io.Platform.script)),
         'google-java-format-1.3-all-deps.jar');
-    final File javaFormatterFile = fileSystem.file(javaFormatterPath);
+    final File javaFormatterFile =
+        packagesDir.fileSystem.file(javaFormatterPath);
 
     if (!javaFormatterFile.existsSync()) {
       print('Downloading Google Java Format...');