core/pkg-generic: add variable to store the package rawname-version

core/pkg-generic: add variable to store the package rawname-version

Introduce a new per-package variable to store the 'rawname-version'
tuple, instead of computing it every time we need it.

Currently, it's only a single location, but follow-up patches will
introduce more use of it.

Reported-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>