Replace all uses of pkg_resources with importlib

[hmpf]

The difficulty is making this work on python 3.7...

$ rg pkg_resources
python/nav/config.py
30:import pkg_resources
251:    for name in pkg_resources.resource_listdir('nav', current_path):
254:        if pkg_resources.resource_isdir('nav', full_name):
275:    content = pkg_resources.resource_string('nav', resource_path)

python/nav/statemon/checker/RadiusChecker.py
17:from pkg_resources import resource_filename

python/nav/buildconf.py
5:import pkg_resources
14:    VERSION = pkg_resources.get_distribution("nav").version
15:except pkg_resources.DistributionNotFound:

python/nav/pgsync.py
29:from pkg_resources import resource_listdir, resource_string

[hmpf]

The stuff in python/nav/config.py, python/nav/statemon/checker/RadiusChecker.py and python/nav/pgsync.py can be replaced with importlib_resources.

[hmpf]

The stuff in python/nav/buildconf.py can be handled by importlib_metadata