Unowned facilities/units as ruins

**What do you want to see?**
An alternate type of ruins which just spawns some particular facility which may or may not be buildable by players on an uninhabited planet. Maybe units too?

**How would it help players?**
Ruins could be destroyed, either intentionally or unintentionally!

**Alternatives**
Can't think of anything

**Screenshots and mockups**
N/A

**Things to consider**
If we do units, where do the designs come from? Do they defend the planet from being colonized or are they inactive?