Maktaba added to runtimepath twice when symlinked and installed manually

Minor issue. When using the manual installation procedure described at https://github.com/google/maktaba/blob/master/vroom/manual.vroom, and Maktaba is symlinked, then it is added to the runtimepath twice, once with the symlink location and once with the resolved symlink. See also #31.

Example vimrc:

```
" ~/.vim -> ~/code/dotfiles/.vim
set runtimepath+=~/.vim/maktaba
```

Resulting runtimepath is `...,/home/glts/code/dotfiles/.vim/maktaba,.../home/glts/.vim/maktaba`.