Allow checking for packages with no reverse dependencies

[thesamesam]

We currently run https://gitweb.gentoo.org/proj/qa-scripts.git/tree/leaf-packages.py?id=30a30507c911d4048000dbbc1a3bc195e960a86e on a cron job daily for qa-scripts.gentoo.org but it's quite slow for something to run daily as there's no caching. It was originally written for https://leo3418.github.io/2021/07/18/find-leaf-packages.html (thanks @Leo3418).

It'd be nice if pkgcheck has something built in for this which included caching.

See also https://github.com/pkgcore/pkgcheck/issues/236.