pkg-download: check for already downloaded file in the download wrapper

Instead of repeating the same test again and again in all our download
rules, just delegate the check for an already downloaded file to the
download wrapper.

This clears up the path for doing the hash checks on a cached file
before the download.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>