Fix rtp plugin detection to ignore .nvim as well as .vim

Neovim uses .nvim/ instead of .vim/ for local vimfiles, causing `maktaba#plugin#RegisteredPlugins` to consider '.nvim' as a plugin name and causing test failures in plugin.vroom and pluginutil.vroom when running vroom in neovim mode.

This changes plugin detection to ignore the .nvim/ dir.