python3: Add script to help find standard library dependencies

python3: Add script to help find standard library dependencies

This adds a script that searches a Python package's source code to find
imports for separately-packaged standard library modules.

The script can be run by calling make with the configure target and
"PY3=stdlib V=s" arguments, e.g.

    make package/python3-lxml/configure PY3=stdlib V=s

This also updates the readme on how to call this script, as well as more
information on Python package dependencies in general.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>