I think we need the concept of optional service dependencies after all.

Consider this AS7 scenario:

Weld subsystem is enabled
Tx subsystem is enabled

Weld is then installed with the Tx service turned on. However consider:

Weld subsystem is enabled
Tx subsystem is not installed

Weld may still begin, but it simply won't start up the Tx service.

Now there are ways we can solve this without optional dependencies, but it would be most elegant if the Weld service could simply look at the injected Tx service, and if it's null, just skip installing it.

Note that if an optional service is not present, adding it in later will not affect the already-registered service even if the service is stopped and started again.