Cabal.Distribution.PackageDescription.Check should be factored per component

[ezyang]

Right now Cabal.Distribution.PackageDescription.Check operates on a sort of ad hoc basis, calling allBuildInfo to implement various checks. This means that there are few cases where we completely forget to validate fields at all, especially build-depends in stanzas. Instead, we should have a function which validates a BuildInfo, and call it on each of the build-infos.

[ulysses4ever]

@ffaf1 could I ask you to take a brief look at this ticket and say if it still makes sense?

[ffaf1]

This will be fixed when #8427 is merged.