[5.7] Make route group namespaces absolute if they start with '\'

[Jack97]

Clone of #23066 - Pushing to the master branch because it's a breaking change.

Currently, you can register routes to point to a fully qualified class name if it starts with a '\' - which will ignore the namespace that would otherwise be inherited from the route group stack:

Route::get('users', '\Foo\Bar\UsersController@index')

This PR adds the same functionality for route groups:

Route::namespace('App\Http\Controllers')->group(function () {
    Route::namespace('\Foo\Bar')->group(function () {
        Route::get('users', 'UsersController@index'); // Foo\Bar\UsersController@index
    });
});

An example use case for this functionality would be for route macros provided by a package. For example:

Route::macro('packageResources', function () {
    Route::namespace('\Vendor\Namespace')->group(function () {
        Route::get('resources', 'ResourcesController@index');
        // Long list of other routes.
    });
});

This means you can use the above macro in your applications main routes file without the namespace being inherited.

[klgd]

Are you planning to merge into 5.6? @taylorotwell

[GrahamCampbell]

No, because this is a breaking change.