When importing an `aws_vpc_peering_connection`, the code assumes that
the account under Terraform control is the initiator (requester) of the
VPC peering request. This holds true when the peering connection is
between two VPCs in the same account, or when the peering connection has
been initiated from the controlled account to another.
However, when the peering connection has been initiated from a foreign
account towards the account under management, importing the peering
connection into the statefile results in values of `peer_vpc_id` and
`vpc_id` being the opposite way round to what they should be, and in the
`peer_owner_id` being set to the managed account's ID rather than the
foreign account's ID.
This patch checks the Accepter and Requester Owner IDs against the AWS
connection's reported owner ID, and reverses the mapping if it is
determined that the VPC peering connection is owned by the foreign
account.