Add export by packagebase for the developer reports

[christian-heusel]

It would be really useful if the developer reports (i.e. https://archlinux.org/devel/reports/old/) would have an export by packagebase endpoint, similar to how todolists have it: https://archlinux.org/todo/gcc-libs-deprecation/pkgbases/packages/

[benmezger]

Hey @christian-heusel I took some time to implement this based on my understanding on how TODOs work. See #662