Symlinked plugin with addon-info.json "name" may be added to the runtimepath twice

This happens on an old MacVim 7.3, but not on newer versions of Vim.

Create a plugin with an `addon-info.json` file with an explicit `"name"`, for example `~/code/vim-linked/addon-info.json`:

```
{
"name": "linked.vim",
"version": "0.0.0",
"dependencies": {
"maktaba": {}
}
}
```

Create a symlink to the plugin location, then start Vim with a vimrc that `#GetOrInstall`s the symlink location. (Here I used locations with different leaf dir names, but the same thing happens when the leaf dir names are the same.)

```
set nocompatible
source ~/.vim/maktaba/maktaba/bootstrap.vim
call maktaba#plugin#GetOrInstall('~/.vim/maktaba/linked')
```

Result. `&rtp`:

```
~/.vim,
~/code/vim-linked/,
~/.vim/maktaba/linked/,
/Applications/MacVim.app/Contents/Resources/vim/vimfiles,
/Applications/MacVim.app/Contents/Resources/vim/runtime,
/Applications/MacVim.app/Contents/Resources/vim/vimfiles/after,
~/.vim/after,
~/.vim/maktaba/maktaba
```

`:echo maktaba#plugin#RegisteredPlugins()`:

```
['linked.vim', 'maktaba']
```