setup: Fix the warning "Package would be ignored"

The warning:
  ############################
  # Package would be ignored #
  ############################
  Python recognizes 'kconfig_hardened_check.config_files.distros' as an importable package,
  but it is not listed in the `packages` configuration of setuptools.

  'kconfig_hardened_check.config_files.distros' has been automatically added to the distribution only
  because it may contain data files, but this behavior is likely to change
  in future versions of setuptools (and therefore is considered deprecated).

  Please make sure that 'kconfig_hardened_check.config_files.distros' is included as a package by using
  the `packages` configuration field or the proper discovery methods
  (for example by using `find_namespace_packages(...)`/`find_namespace:`
  instead of `find_packages(...)`/`find:`).

So let's use "find_namespace:" for package directory to include
the package data. More info in the documentation:
https://setuptools.pypa.io/en/latest/userguide/package_discovery.html#finding-namespace-packages