List/get needed modules in `setup.cfg`

Don't know if this is easy, but it would seem that the list of modules for which to list versions should just be gotten from `setup.cfg` - it could then default to `install_requires` or something else that is sensible.

Read setup.cfg first, then trace how the plugin currently determines which modules receive version information. Confirm whether install_requires is the intended source before changing the behavior; done means the reported module versions follow the configured dependency list.

Written by the indexing model from the issue text.