Move submodule-specific settings into submodules using form_alters, instead of privileging on settings form

If a project requires only the saleforce client service, enabling 'saleforce' module and visiting the "Salesforce Settings" configuration form throws the following exception:

Symfony\Component\Routing\Exception\RouteNotFoundException: Route 'salesforce_push.endpoint' does not exist.

This is due the form trying to build that route on:

$standalone_url = Url::fromRoute(
        'salesforce_push.endpoint',
        ['key' => \Drupal::state()->get('system.cron_key')],
        ['absolute' => TRUE]);

while the salesforce_push module is not enabled.

We may want to wrap part of all the SettingsForm::buildForm() code with ModuleHandler::moduleExists().