pw_build: Integrated actions for pw_python_script

- Support adding an action to a pw_python_script. This bridges the gap
  between pw_python_script and pw_python_action and prevents
  accidentally creating an action that lacks dependencies on its
  pw_python_script target.
- Require either script or module in pw_python_action.
- Ensure pw_python_actions rerun when any files in their python_deps
  change, even if the packages do not have to be reinstalled.

Change-Id: I104bc73b63293b61a0a47e0dcc12f1595f8d4d35
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/43740
Commit-Queue: Wyatt Hepler <hepler@google.com>
Reviewed-by: Keir Mierle <keir@google.com>