Fix bogus error message for the path used to reference sub packages.
Fix bogus error message for the path used to reference sub packages.

Sub packages should ("must") always be referenced through the path of their base package instead of their own path (in case it differs). However, the dependency resolution code assume the opposite. Both way are now accepted, but referencing the sub package path directly results in a warning.

rjframe/dub,Geod24/dub,D-Programming-Language/dub