Changing the directory where external tarballs are saved

[Khaled Hosny]

Hi all,

I proposed a change on gerrit[1] to save downloaded external tarballs
under external/tarballs instead of the misleading src/ that we use now.
Please raise your objections if you have any.

Currently the existing src/ directly does not get renamed, so people
will have to move it manually to avoid needless re-downloads.

Regards,
Khaled

1. https://gerrit.libreoffice.org/6597