Divorce splitting vehicles and adding wreckage tag

[irwiss]

Summary

None

Purpose of change

Fixes #61297
Fixes #58427 - on a limb here, but looks like stuff "behind" the vehicle in the screenshot is a racked vehicle, and likely fake parts were disabled for this vehicle

Any existing vehicles tagged "wreckage" are write-offs unless someone thinks up a method to untag just the non-wrecks...

Building a vehicle starting from a wrecked vehicle also looks to be a way to turn off fake parts for a specific vehicle 🤔

Describe the solution

Change a few functions to move adding "wreckage" tag to wreckage handling, allowing bike racks use vehicle::split_vehicles without applying the tag

Also removes a couple function overloads, used once, added extra code and needed more maintenance than their worth

Describe alternatives you've considered

I'm not sure why wrecks don't get fake parts - it's not possible to repair wrecks into non-wrecks as the flag is permanent for an instance of a vehicle.
Tests pass even if wrecks do get fake parts, considered removing the special casing, but there's no explanation for this in the fake parts PR, so keep on cargo culting.

Testing

Scenario in #61297 works for testing

Additional context