scripts: west_commands: decouple runners pkg from west

I've had some requests to be able to use code in the runners package
without having west installed.

It turns out to be pretty easy to make this happen, as west is
currently only used for west.log and some trivial helper methods:

- To replace west log, use the standard logging module
- Add an appropriate handler for each runner's logger in
  run_common.py which delegates to west.log, to keep
  output working as expected.

Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>