Description of problem:

'dnf download' does not save the downloaded rpm on disk

Version-Release number of selected component (if applicable):
libdnf-0.10.1-1.fc26.x86_64
dnf-2.7.2-1.fc26.noarch
dnf-plugins-core-2.1.4-1.fc26.noarch

How reproducible:

Always, also with '--sources'

Steps to Reproduce:
1. mkdir /tmp/dnf
2. dnf download --source --destdir /tmp/dnf vim
3. ls /tmp/dnf

Actual results:

/tmp/dnf is empty:

$ ls -l /tmp/dnf/
total 0

Expected results:

the source rpm file for vim


Additional info: