Replace deprecated pkg_resources check

[rozyczko]

Describe the bug
Class PackageGatherer uses pkg_resources.working_set to poll the installed packages.

This package has been deprecated ( https://setuptools.pypa.io/en/latest/pkg_resources.html ) and we should replace this with

import importlib.metadata
installed_packages = [x.name for x in importlib.metadata.distributions()]

SasView version (please complete the following information):

• Version: <=6.0

Operating system (please complete the following information):

• OS: all

Additional context
Errrr where do we actually use this functionality? Nothing in the code seems to call the relevant methods.

[krzywon]

This was added for 5.0.5 in #1961 but was removed from the GUI because the release version never worked properly. We might want to revisit this now the cli is working.