Snapcraft now supports a special `core` base for modern snaps to target
on which has the same effect of using the Ubuntu 16.04 based `core` snap
in the past(it is special as the intended base snap to support Ubuntu
16.04 is rather the `core16` base snap, but it isn't ready yet).
This patch migrates the gallery-dl snap to `core` base, which allows the
user to avoid installing the additional `core18` base snap in order to
run it. It also drops the redundant stage-packages entries that was
collected by Snapcraft automatically in the first place.
Signed-off-by: 林博仁(Buo-ren Lin) <Buo.Ren.Lin@gmail.com>
2df3aaf966