Fix/improve how PACKAGE setting is initialized
Fix/improve how PACKAGE setting is initialized

- Make `derive_top_level_package_name()` return the first segment of a
  dotted module path
- Fix `derive_top_level_package_name()` docstring; previously, it
  incorrectly stated that the `level` arg could be used to account for
  being called from a sub-package
- Move comments about package structure from
  `derive_top_level_package_name()` docstring to `init_settings()`
  because it's more useful and relevant there

wylee/django-arcutils,wylee/django-arcutils,PSU-OIT-ARC/django-arcutils,PSU-OIT-ARC/django-arcutils