eZ\Publish\Core\SignalSlot\Slot and eZ\Publish\Core\SignalSlot\Signal should not be marked as @internal as they are clearly to be used https://ez-systems-developer-documentation.readthedocs-hosted.com/en/master/guide/repository/#signal-slots

Currently these cause depreciation warnings in the symfony debugger and logs.

Should these be moved to SPI instead of in Core?