[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/license_check_command.dart

@@ -98,11 +98,10 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 class LicenseCheckCommand extends PluginCommand {
   /// Creates a new license check command for [packagesDir].
   LicenseCheckCommand(
-    Directory packagesDir,
-    FileSystem fileSystem, {
+    Directory packagesDir, {
     Print print = print,
   })  : _print = print,
-        super(packagesDir, fileSystem);
+        super(packagesDir);
 
   final Print _print;