src/Installer/Form/ModuleConfigureForm.php
public function submitForm(array &$form, FormStateInterface $form_state) {
    $installModules = [];

    foreach ($form_state->getValues() as $key => $value) {

      if (strpos($key, 'install_modules') !== FALSE && $value) {
        preg_match('/install_modules_(?P<name>\w+)/', $key, $values);
        $installModules[] = $values['name'];
      }
    }

    $buildInfo = $form_state->getBuildInfo();

    $install_state = $buildInfo['args'];

    $install_state[0]['thunder_additional_modules'] = $installModules;
    $install_state[0]['form_state_values'] = $form_state->getValues();

    $buildInfo['args'] = $install_state;

    $form_state->setBuildInfo($buildInfo);

  }
public function submitForm(array &$form, FormStateInterface $form_state) {
    $installModules = [];

    foreach ($form_state->getValues() as $key => $value) {

      if (strpos($key, 'install_modules') !== FALSE && $value) {
        preg_match('/install_modules_(?P<name>\w+)/', $key, $values);
        $installModules[] = $values['name'];
      }
    }

    $buildInfo = $form_state->getBuildInfo();

    $install_state = $buildInfo['args'];

    $install_state[0]['thunder_additional_modules'] = $installModules;
    $install_state[0]['form_state_values'] = $form_state->getValues();

    $buildInfo['args'] = $install_state;

    $form_state->setBuildInfo($buildInfo);

  }
{@inheritdoc}