fix(build): avoid trying to download none existing debug packages

Since last release, offload building now has proper error handling enabled. This unfortunately lead to a regression for packages, like any packages, where makepkg claims debug packages are available during --packagelist while none were actually built. This leads to the scp command failing when trying to download the none existing packages which ultimately leads to a termination of the build script.

Fix this by filtering out package files that do not exist before trying to download them.

Fixes #208 (closed)

Component: pkgctl build

Signed-off-by: Levente Polyak anthraxx@archlinux.org