[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/publish_check_command.dart

@@ -21,13 +21,12 @@ import 'common.dart';
 class PublishCheckCommand extends PluginCommand {
   /// Creates an instance of the publish command.
   PublishCheckCommand(
-    Directory packagesDir,
-    FileSystem fileSystem, {
+    Directory packagesDir, {
     ProcessRunner processRunner = const ProcessRunner(),
     this.httpClient,
   })  : _pubVersionFinder =
             PubVersionFinder(httpClient: httpClient ?? http.Client()),
-        super(packagesDir, fileSystem, processRunner: processRunner) {
+        super(packagesDir, processRunner: processRunner) {
     argParser.addFlag(
       _allowPrereleaseFlag,
       help: 'Allows the pre-release SDK warning to pass.\n'