Detect unused BuildRequires

[yselkowitz]

The mock unbreq plugin provides a means of determining if a specified BuildRequires is not actually needed. (This often arises when something was a static BR in a previous version, but subsequent changes have rendered it unused.)

The question is how to do this. It doesn't seem like COPR has any options that would allow for this, and I'm not sure we could enable this in koji itself. However, if we can find a way to run it (either periodically, or an ongoing basis) then we might be able to uncover some stale build deps that could be pruned.

[xsuchy]

We plan to enable this plugin in Copr soon. I will add here a link to tracking issue once I will have it.