When two rules differ only in source security group, EC2 APIs return
them as a single rule, but Terraform requires separate
aws_security_group_rule resources.
6bdab07174 changed Read to set source_security_group_id (and
cidr_blocks) from the rule returned from EC2 and chose the first
source_security_group_id arbitrarily, which is wrong.
Makes TestAccAWSSecurityGroupRule_PartialMatching_Source pass again.
Also adds a comment noting that there is a bug in the new resource
importing feature.
Fixes#6728.
Clint