Move some more core hooks out of system.api.php

On #2299715: [meta] Move core hooks from system.api.php to core.api.php or other files, we're making sure that core/modules/system/system.api.php only contains hooks that are specifically related to the System module.

This issue's tasks:

a) Move the two cache-related hooks to core.api.php [ hook_cache_flush(), hook_rebuild() ] -- make sure they are within the @addtogroup hooks section of the file.

b) Move hook_link_alter() to menu.api.php

c) Move the two config hooks to core.api.php [ hook_config_import_steps_alter(), hook_config_schema_info_alter() ]