New issue by xtraeme on void-packages repository

https://github.com/void-linux/void-packages/issues/21044

Description:
```
[juan@leysa xbps-fbulk]$ cat fbulk-202004161057/bad/live555.txt
=> live555-2019.05.12_1: building [gnu-makefile] for x86_64-musl...
   [runtime] live555-2019.05.12_1: not found (self, ignored)
   [runtime] live555-2019.05.12_1: not found (self, ignored)
=> live555-2019.05.12_1: running do-fetch hook: 00-distfiles ...
=> live555-2019.05.12_1: fetching distfile 'live.2019.05.12.tar.gz'...
http://www.live555.com/liveMedia/public/live.2019.05.12.tar.gz: Not Found
[juan@leysa xbps-fbulk]$
```

The new code just adds all `depends` from subpkgs without taking into account duplicates.