Verify that we retain package names even if all versions are dropped

[thomashoneyman]

The registry's initial contents are imported from the existing Bower registry. During that import we do not accept all package versions, for various reasons as described in #300. Some packages have issues with all versions. We need to verify that packages in which all versions have failed are still included in the resulting registry index. This is necessary because it is a security issue to allow an existing package name to be re-registered.