Be more tolerant about plugin name conflicts

Adding two plugins with the same canonical name to `&rtp` breaks Maktaba's plugin detection completely, reporting e.g.

```
Error detected while processing function maktaba#plugin#Detect[2]..maktaba#plugin#GetOrInstall:
line 11:
E605: Exception not caught: ERROR(AlreadyExists): Conflict for plugin "foo": foo/ and foo.vim/
```

However, it seems like we should be able to deal with this more tolerantly and ignore the second one, not just completely fail.

(The real case I'm looking at is using both https://github.com/junegunn/fzf and https://github.com/junegunn/fzf.vim, which seems reasonable.)