core/download: look for archives in the global download dir first

core/download: look for archives in the global download dir first

For existing setups, the global donload directory may have a lot of the
required archives, so look into there before attempting a download.

We simply hard-link them if found there and not in the new per-package
loaction. Then we resume the existing procedure (which means the new
hardlink will get removed if it happened to not match the hash).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>