Support installing local deb packages

[d-loose]

There's no support for dealing with local packages yet. However 'snap-store' is supposed to handle those by default:

$ grep debian /usr/share/applications/defaults.list 
application/vnd.debian.binary-package=snap-store_ubuntu-software-local-file.desktop
application/x-debian-package=snap-store_ubuntu-software-local-file.desktop

Do we have an alternative way of handling those for the time being?