>self-driving cars could cooperatively pack/unpack themselves into space efficient configurations.

I've had this same thought, but haven't seen it discussed much.

Parking lots essentially switch from random access (which requires expensive unoccupied 'lanes') into a slowly snaking space-filling curve. Cars can crawl slowly from entrance to exit, moving like water through a pipe. The paths can even branch and merge to make efficient use of the entire surface area.

If you also want to charge the cars stored in the parking lot, your solutions fall into two broad categories:

1.) Arrange the parking lot with rows of chargers that automatically plug in. Cars choose the appropriate row based on state-of-charge, twith he algorithm making sure the entire row finishing charging at roughly the same time. This minimizes unnecessary disconnect/move forward/reconnect cycles. Or,

2.) put wireless charging under the parking lot. This would be a lot more feasible than electrifying large swaths of highway.

Personally I still prefer option 1, because the peak charging speeds can be much greater.