hcl2template: report localtion for cycle detection

When a datasource fails to be evaluated because a cycle has been
detected, we point out one of the links of the chain now so that users
have a better idea of what to look at.

hcl2template/types.packer_config.go

@@ -368,6 +368,7 @@ func (cfg *PackerConfig) recursivelyEvaluateDatasources(ref DatasourceRef, depen
 				"sources. Either your data source depends on more than ten " +
 				"other data sources, or your data sources have a cyclic " +
 				"dependency. Please simplify your config to continue. ",
+			Subject: &(cfg.Datasources[ref]).block.DefRange,
 		})
 		return dependencies, diags
 	}