core/pkg-infra: don't display "foo Downloading" when there's no source

core/pkg-infra: don't display "foo Downloading" when there's no source

Commit ea55e1323 (core/pkg-infra: don't enforce site-method for extra
downloads) forgot to account for those packages that have nothing to
download, like the skeleton, or like virtual packages...

The side effect is that the message "foo Downloading" is thus
displayed when it should not be.

Fix that.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>