Second try of #6350

Fixes the UnusedImportRemovingPostRector rule when the imported namespace doesn't match the letter case in the code, by comparing all names in lowercase, except consts, because of their case sensitivity.

Additionally:
Early return for cases when no usage of imported namespaces has been found.

Example:

<?php

use Json;


class Json {
}

var_dump(JSON::class);

https://getrector.com/demo/f2508805-2d69-4792-908b-3555589439e1

To do:

• add a test with the mixed namespace for consts