Better handling of dependencies

[uliska]

If a package loads another one (i.e. depends on it) that isn't present currently there's only a warning about a failure to load the secondary package. Users are not pointed to the fact that this is a dependendy issue.

There should be sufficient information available to detect that fact (from the package.cnf) and issue an explicit warning (error?) instead.

See openlilylib/scholarly#70