allow virtual packages to be versioned

Presently, you can use apk add -t package-name to inform the package manager that you want to create a virtual package. These virtuals are visible to SCA tools and security scanners.

However, you cannot specify the version of a package, which is unfortunate, as it would be a way to allow people to record the packages they build themselves outside apk in the apk database, so that SCA tools and security scanners can see them.