Bikeshedding "recompilation avoidance"-avoidance!

[merijn]

As brought up (I think mostly by me? ;)) in #4247 and #5287 new-build aggressive caching (and my attempts to make it even more so) leads to a problem where you want things to be rebuild, but cabal-install decides it doesn't have to. For example, adding new warning flags or debug dump flags, and perhaps even more reasons I'm not considering right now.

As a result, I think we some UI/flag to force cabal-install to rebuild something. I think we'd want two flags, one that just rebuilds every local package and one to specify 1 or more local packages to be rebuild. However, I'm terrible at naming things and maybe there's other cases we'd like covered too, so please bikeshed away.