scripts: make runner a west subpackage

This is a stepping-stone to adding runner functionality into west
itself.

Since all of the runner tools assume a Zephyr build directory layout,
this doesn't put anything generic into a Zephyr-specific tool.

Make minimal adjustments to zephyr_flash_debug.py to keep existing
build system targets working unmodified.

Signed-off-by: Marti Bolivar <marti@opensourcefoundries.com>