Fix incorrect pydcs import paths.

I've been wrongly importing these from `pydcs.dcs` instead of just
`dcs`, because that was what PyCharm thought they were. These will all
be broken when we get back to using a real pydcs instead of relying on
its directory being in our tree.

This page in the wiki should be updated:
https://github.com/Khopa/dcs_liberation/wiki/Developer's-Guide

Instead of recommending that `PYTHONPATH` be updated in the run
configuration, it should instead recommend that Settings -> Project:
dcs_liberation -> Project Structure be set to exclude the pydcs
directory from the dcs_liberation content root, and add the pydcs
directory as a *separate* content root.

Alternatively, we could recommend that configure a virtualenv (good
advice anyway, and pycharm knows how to set them up) that have people
run `pip install -e pydcs`.

I think even easier would be switching from the virtualenv-style
requirements.txt to pipenv, which can actually encode the `-e` style
pip install into its equivalent of requirements.txt.

gen/airfields.py

@@ -7,7 +7,7 @@ from dataclasses import dataclass, field
 import logging
 from typing import Dict, Optional, Tuple
 
-from pydcs.dcs.terrain.terrain import Airport
+from dcs.terrain.terrain import Airport
 from .radios import MHz, RadioFrequency
 from .tacan import TacanBand, TacanChannel