core/download: fix source check for git method

Running foo-source-check on packages retrieved from git fails.

This is because there is no associated stamp file, so we do not have a
rule-assignment that sets PKG for foo-source-check.

But it does not make sense to have a stamp file at all, because
source-check is not supposed to change anything: the status after is
exactly the same as before; nothing is downlaoded, so there is no
progress (whatsoever) to memorise.

Fix that by just defining PKG in the source-check rule definition.

Fixes #9796.

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