Apt-get commands from within a deb postinst

It is not possible to invoke an APT command (apt-get, aptitude..) from within a package script (preinst, postinst, prerm, postrm...).

Enabling so would raise lots of problems, especially for dependency and ordering of package installation.

Various workaround have been used, either by using proper package (pre) dependencies or by providing an easy-to-use tool for your users (like module-assistant and other tools).

In your case, your package could just conflict with popularitycontest to uninstall it. Also, if your user have "your" package, it means they have already added an entry to their sources.list, so they can add another one!