Laravel 5.4 - How to override route defined in a package?

In config/app.php in the providers array put the service provider of the package before App\Providers\RouteServiceProvider::class, and then in your web.php routes you'll be able to override it with your custom route.

EDIT For Laravel package auto discovery you can disable the package being auto discovered in your composer.json like this:

"extra": {
    "laravel": {
        "dont-discover": [
            "barryvdh/laravel-debugbar"
        ]
    }
},

In this example the barryvdh/laravel-debugbar package won't be autodiscovered, which means you would have to manually include its service provider in the config array and then you'll be able to rearrange your custom provider in the desired order.

Another option -- which doesn't have to muck with the order of service providers -- is to add a binding for the controller. So e.g. in AppServiceProvider,

$this->app->bind(
    \Vendor\Package\Controllers\Auth\LoginController::class,
    App\Controllers\Auth\LoginController::class
);

You'll have to match controller method names, but you're doing that already in your example.

(Caveat on this answer: I haven't tested it in Laravel 5.4, but I just did this in Laravel 6.0 using the $bindings property which was added in Laravel 5.6. That said, this should be correct 5.4 syntax for doing the same thing).

Edit: For Laravel 6+ you can instead add the binding to the bindings array in AppServiceProvider:

public $bindings = [
    \Vendor\Package\Controllers\Auth\LoginController::class =>
        App\Controllers\Auth\LoginController::class,
    // other bindings
]