[5.1] Fix app namespace determination

[arrilot]

Closes #8997

There are some problems with app namespace determination that was introduced in #8084

1. Nested namespaces doesn't work atm, but in L5.0 they work fine.
2. In some rare cases kernel classes can be overwriten since they are bound into the container in laravel/laravel https://github.com/laravel/laravel/blob/develop/bootstrap/app.php

So I guess it's better to return to the code we have in 5.0, sorry.

[GrahamCampbell]

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