Issue #2126421 by slashrsm: Decouple \Drupal\Core\Path\AliasManager and \Drupal\Core\Path\Path.

core/core.services.yml

...Drupal/Core/CacheDecorator/AliasManagerCacheDecorator.php

core/lib/Drupal/Core/EventSubscriber/PathSubscriber.php

core/lib/Drupal/Core/Path/AliasManager.php

core/lib/Drupal/Core/Path/AliasManagerInterface.php

core/lib/Drupal/Core/Path/Path.php

...modules/system/lib/Drupal/system/Tests/Path/AliasTest.php

...stem/lib/Drupal/system/Tests/Routing/MockAliasManager.php

core/modules/system/system.module