[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/version_check_command.dart

@@ -73,15 +73,13 @@ Map<Version, NextVersionType> getAllowedNextVersions(
 class VersionCheckCommand extends PluginCommand {
   /// Creates an instance of the version check command.
   VersionCheckCommand(
-    Directory packagesDir,
-    FileSystem fileSystem, {
+    Directory packagesDir, {
     ProcessRunner processRunner = const ProcessRunner(),
     GitDir gitDir,
     this.httpClient,
   })  : _pubVersionFinder =
             PubVersionFinder(httpClient: httpClient ?? http.Client()),
-        super(packagesDir, fileSystem,
-            processRunner: processRunner, gitDir: gitDir) {
+        super(packagesDir, processRunner: processRunner, gitDir: gitDir) {
     argParser.addFlag(
       _againstPubFlag,
       help: 'Whether the version check should run against the version on pub.\n'
@@ -117,7 +115,7 @@ class VersionCheckCommand extends PluginCommand {
     const String indentation = '  ';
     for (final String pubspecPath in changedPubspecs) {
       print('Checking versions for $pubspecPath...');
-      final File pubspecFile = fileSystem.file(pubspecPath);
+      final File pubspecFile = packagesDir.fileSystem.file(pubspecPath);
       if (!pubspecFile.existsSync()) {
         print('${indentation}Deleted; skipping.');
         continue;