[2.x] Add namespace check to Auth routes

[barryvdh]

When removing the Route default namespace (as is default in Laravel 8), the Auth routes will break. This is the case for new Laravel 8 apps, but also when you want to upgrade to match the new defaults.

I understand this package is now deprecated, but if this is the only change it might be worth considering to give users the time to upgrade/change/get comfortable to Jetstream and let it stabilize/grow a bit first.

This checks if the Class with namespace exists. If it doesn't, it adds the default namespace, to compensate for laravel/laravel@58a98ef#diff-b467cc786635bd6945b17726282f1c64

Fixes #138 #141 #142

[barryvdh]

It's easier to check without whitespace changes: https://github.com/laravel/ui/pull/143/files?w=1

[driesvints]

@barryvdh great solution. Thanks 👍

[dimnsk]

I have not Auth\LoginController but I have the value in the field RouteServiceProvider.namespace = "App\Http\Controllers" so namespace is applied twice and error appeared:

Illuminate\Contracts\Container\BindingResolutionException
Target class [App\Http\Controllers\App\Http\Controllers\Auth\ForgotPasswordController] does not exist.