[5.4] Allow route groups to be loaded from a file directly

[JosephSilber]

Since keeping the route definitions for each group in its own file is a common pattern, this PR allows the file path to be passed directly to the group method on the router.

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Will convert this:

Route::prefix('api')
     ->middleware('api|auth:api')
     ->namespace($this->namespace)
     ->group(function ($router) {
         require base_path('routes/api.php');
     });

into this:

Route::prefix('api')
     ->middleware('api|auth:api')
     ->namespace($this->namespace)
     ->group(base_path('routes/api.php'));

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This is another attempt at #14572

[CristianLlanos]

Nice one, @JosephSilber. Thanks 👍