The fix for gh-1475 introduced the use of an artifact's group to
discriminate between two libraries with the same name (artifact id)
and version. However, in the case of Gradle, a group name was not
provided for libraries that have been resolved from a repository.

This commit updates ResolvedArtifactLibrary to use the group obtained
from the underlying ResolvedArtifact as its discriminator.

Fixes gh-1543