Fix detection of invalid dependencies in sub packages of the root package. See #334.
Disallow :short_hand references to sub packages from within sub packages. See #334.

Until the semantics are defined, disallow this use of the short hand syntax. The two
semantic possibilities for ":subpack2" within a sub package "pack:subpack1" are either "pack:subpack2" or "pack:subpack1:subpack2". Both could be more or less valid options.