More efficient handling for cleaning up orphan packages from a project

[darix]

Is your feature request related to a problem? Please describe.

For large projects it becomes unfeasable to track which packages are still needed for building or installing packages.

• devel:languages:ruby:extensions
• distro repositories like openSUSE:Factory

the distro release team wrote themself a script which tries to do this but it isnt very efficient.

Describe the solution you'd like

an OBS request for the backend that allows me to pass in a list of packages (maybe also patterns) that should be available
and the OBS would return a list of all packages which are not needed for building or installing those packages.

Given we already have solve files in the backend this should be way more efficient than doing it via the current methods.

Describe alternatives you've considered

script that walks over the whole project with osc whatdependson and checking the repodata for runtime deps.
especially the first part is inefficient.