bug#15229: guix download does not understand https

[Andreas Enge]

The title says it all:

guix download https://fedorahosted.org/releases/x/m/xmlto/xmlto-0.0.25.tar.bz2

results in

ERROR: missing interface for module (gnutls)
failed to download "guix-file.Oty9Cl" from 
"https://fedorahosted.org/releases/x/m/xmlto/xmlto-0.0.25.tar.bz2";
guix download: error: 
https://fedorahosted.org/releases/x/m/xmlto/xmlto-0.0.25.tar.bz2: download 
failed

(under guile 2.0.5-deb+1-3).

Since guix build -S works on the same file, something needs probably to be
ported from guix build to guix download.

Andreas