Fix: only count distance traveled in vehicles for cargo payment (#11283)

No longer you can utilize the free (and instant) labour of station
workers, transporting your cargo from one part of the station to
the other. No more!

Based on patch by dP.

Codechange: cleanup CargoPacket in terms of variable/function names (#11278)

Over the years, things got reused and changed, making the current
names somewhat unclear in what they actually mean and do.