Comment 8 for bug 616685

Quoting Stefano from IRC:

"you can look at apache2.2-common's default-site installation method, as this
was inspired by apache2.2-common's old method
basically, don't have the deb install stuff there, but if there's nothing there
(fresh install) or old default site, you can copy a default site in"

By checking apache2.2-common package, I find that they install the default site
somewhere in /usr/share/<package>, then in the postinst they do some check, if the
conditions pass, they would copy that default site to /var/www/