debexpo: adding "Already in Debian" field for packages list

I've sent a merge request to show "Already in Debian" column in packages list on mentors.debian.net.

salsa.debian.org

At first, I've used Emoji, but for consistency, it has been modified to use "Yes or No".

This feature is not fully merged yet, but it may be useful to distinguish "This package is already in Debian or not" for sponsor.

f:id:kenhys:20201108151153p:plain
Aready in Debian