- Introduce new system for registering plugins with the pw command.
Plugins can be registered for a directory tree by listing them in a
PW_PLUGINS file.
- Update most existing plugins to be registered as "built-in" plugins in
pw_cli.
- Register pw_presubmit using a PW_PLUGINS file, since it only applies
to the Pigweed repository.
- No longer add plugins to the pw argument parser as subparsers.
Instead, use argparse to separate the pw arguments from the plugin
arguments.
- Remove __init__.py files that were only used to trigger plugin
registration, since __init__.py is not required in Python 3.
Change-Id: I20028f2053476df2afebd36e9299062e7e96727e
