Debian Package Rules as Mercurial repositories

[Dominic]

Dear List

I'm looking for the Debian package rules. I could download the source 
file from http://nginx.org/packages/mainline/ubuntu/pool/nginx/n/nginx/. 
But I guess there is an non public repository, where the package rules 
are stored to build all the provided packages.

Something like https://github.com/hhvm/packaging (Package rules for 
HHVM) would be nice to have.

The reason for my question is, that I need to build nginx an older 
version of nginx, but the source package of this older version is not 
longer hosted on http://nginx.org/packages/.
A solution to this problem might be an archive of the nginx builds at 
http://nginx.org/packages/ or a public repository for the build process.

Regards

Dominic