distutils-r1.eclass: Warn if *-nspkg.pth files are installed

distutils-r1.eclass: Warn if *-nspkg.pth files are installed

Add a check for *-nspkg.pth files indicating implicit setuptools
namespace hack. While they kept namespaces somewhat working without
requiring explicit support in ebuilds, they were unreliable. They
frequently required additional hacks (distutils_install_for_testing) to
get the tests working, and they have proven even more broken for Python
3.5+.

For this reason, those files were deprecated in favor of proper,
explicit namespace support. If they are found to exist, the developer
should ensure to remove them to avoid issues.