.d is a common Unix include directory pattern. See cron.d, xinetrc.d, conf.d etc. Packages can just add/remove things to the include dir rather than modifying the main config.

I do that in my dotfiles too:

The main include:

https://github.com/skx/dotfiles/blob/master/.bashrc

The actual dotfiles:

https://github.com/skx/dotfiles/

(People usually copy the literate emacs configuration ..)