Debian package doesn't check group

#### Overview of the Issue

We are overtaking servers where consul has been installed a long time ago. The user `consul` is there but not the `consul` group. I expected the package to create the group but it doesn't.
The `consul.preinst` script checks if the `consul` user is present. If not, it creates the user+group. If not, it does nothing.

I wanted to submit a PR but I can't find where are the debian package sources. If someone can provide a link, I'll be glad to submit a PR for this bug.

#### Reproduction Steps

Steps to reproduce this issue, eg:

1. Have a server with a consul user but no consul group
2. Install consul debian package (1.8.4-3 here)
3. `grep consul /etc/group` is empty

### Operating system and Environment details

Debian 10

Thanks!

Start by locating the Debian package sources and reading the consul.preinst script mentioned in the report. Reproduce the case with a consul user but no consul group, then install the package and check /etc/group. Done means the package handles this existing-user case so that the consul group is present.

Written by the indexing model from the issue text.