Move car.capnp to opendbc

[sshane]

@deanlee interested in this? We want to replace the dataclasses with car.capnp again, but it can live in opendbc. The duplication of the structs is starting to get annoying and is error-prone if one place is forgotten to be updated.

There are some issues we need to resolve:

• OnroadEvent should be in openpilot but is used in carState.events (deprecated field now). We can make a copy of that and call the struct OnroadEventDEPRECATED.
• log.capnp uses some structs from car.capnp such as radar errors, audble alerts, safety model. We can duplicate most, except the safety model shouldn't need to be edited in both places. Maybe we should turn it into an int from pandaStates? Or can we just include the opendbc struct from cereal and call it a day?
  Edit: I don't think so, you will get a duplicate ID error when you import both car.capnp in opendbc and the duplicate included one in log.capnp (from a symlink)

[deanlee]

commaai/opendbc#1288

The approach of this PR is to move car.capnp to opendbc/car while keeping all other capnp files unchanged. In opendbc, the following code is used to avoid the duplicate ID error:

try:
  from cereal import car
except ImportError:
  capnp.remove_import_hook()
  car = capnp.load(os.path.join(OPENDBC_CAR_PATH, "car.capnp"))

This ensures that car.capnp can be imported properly without conflicts