I have split my .vimrc into several files and placed them into ~/vimfiles/vimrc.d/.
~/vimfiles/vimrc.d/
Currently I source each file in that directory using exact name:
The example from DrAl did not work for me, this is how I achieved it:
for fpath in split(globpath('~/.vimrc.d/', '*.vim'), '\n') exe 'source' fpath endfor