Sanitize and warn about conflicting entries in different `lib+feature` packages

!86 (merged) and d6cab54b made it possible for Architecture and Multi-Arch to be configured from debcargo.toml. The downside of that is those fields can now have different values in feature packages when features are not collapsed, which should probably be avoided. Additionally, when features are collapsed, only a select set of fields in the lib+feature packages should have different values than the main lib package. At the very least, we should warn the user about these differences.