Move Debian packaging into debian/ and mark as overlay repo (Closes: #1094666)

Not placing the Debian packaging files under debian/ breaks most standard Debian tools, for example dch, wrap-and-sort, gbp and Salsa CI.

This commit moves the packaging into the debian/ directory and marks the Git repository as an "overlay" (i.e. a repository that does not contain the upstream sources).

Please note that this MR depends on (and includes) MR !1.