Fixes #1616. Fixes the "latest snapshot" chain resolver

The "latest snapshot" chain resolver was assuming that there's at least
one artifact per module. I think that was the root cause of #1616.

librarymanagement/src/main/scala/sbt/internal/librarymanagement/ivyint/SbtChainResolver.scala

@@ -193,15 +193,11 @@ private[sbt] case class SbtChainResolver(
               // Now that we know the real latest revision, let's force Ivy to use it
               val artifactOpt = findFirstArtifactRef(rmr.getDescriptor, dd, data, resolver)
               artifactOpt match {
-                case None if resolver.getName == "inter-project" => // do nothing
-                case None if resolver.isInstanceOf[CustomMavenResolver] =>
-                // do nothing for now....
-                // We want to see if the maven caching is sufficient and we do not need to duplicate within the ivy cache...
-                case None => throw new RuntimeException(s"\t${resolver.getName}: no ivy file nor artifact found for $rmr")
                 case Some(artifactRef) =>
                   val systemMd = toSystem(rmr.getDescriptor)
                   getRepositoryCacheManager.cacheModuleDescriptor(resolver, artifactRef,
                     toSystem(dd), systemMd.getAllArtifacts.head, None.orNull, getCacheOptions(data))
+                case None => // do nothing. There are modules without artifacts
               }
               rmr
           }