pkresource._files should not fine root_module twice

[robnagler]

Something like this:

    for p in list(
        map(
            lambda m: pkinspect.root_package(importlib.import_module(m)),
            packages
        ) if packages
        or [
              pkinspect.root_package(
                  caller_context if caller_context else pkinspect.caller_module()
              )
          ],
    ):