pkgset: Hardlink downloaded packages
When pungi-gather (or old pungi) download the packages from repos into
work/$arch/, they are linked to work/global/. This was using link_type
However if that is set to symlink, we get a relative symlink in
work/global/ which is later copied under the compose/ directory. Since
it's a relative symlink, it gets broken by this.
The fix is to hardlink the downloaded packages instead in the first
step. Since both the source and destination are in work/ directory, we
know it's the same volume and hardlinks should work. There is a fallback
to copy just to be sure, but it shouldn't ever be used.
Signed-off-by: Lubomír Sedlář <email@example.com>