package: make package download from Mercurial repositories more robust

Previously, when cloning a Mercurial repository,
aborting the clone could (in some cases) leave behind an empty directory.
As a result, a later clone would fail.

Signed-off-by: Mathias De Maré <mathias.demare@gmail.com>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>