Handle "stable" version in registry root

https://issues.sonatype.org/browse/NX-632

Handle packages that only list a "stable" version in the registry root.
The underscore package is one such package that does not list a "latest"
version, but only a "stable" version. Here is the relevant json:

"versions" : {
    "1.6.0" : "stable"
}

This was causing the logic in PackageRoot.wrapVersions to not create any wrapped versions for underscore, and so it was interpreted as unpublished and not as incomplete.