Refactoring phase3 to allow for single-resource validation

[msdemlei]

What I'm after here is being able to write

import pprint

from benson.config import Settings
from benson.oai import phase3

settings = Settings.from_env()
with open("zw.xml", "rb") as f:
  pprint.pprint(
    phase3.validate_one_voresource(f.read(), True, settings))

This PR should do that, but my pytest somehow clashes with my venv setup ("No module named 'benson'"), and I'm too lazy to work out why they fight. Hence, I'm offering this without having run the tests.