fix: declare Lambda's dependency on log group

[bendrucker]

What does this PR do?

Makes explicit the implicit dependency between the Lambda and the CloudWatch log group with name = format("/aws/lambda/%s", var.name).

Motivation

This pattern of hardcoding the Lambda CWL naming scheme with depends_on is used to have Terraform manage a CloudWatch Logs log group for a Lambda. Lambdas don't take a log group, but if one already exists with the right name they'll just use it.

Without depends_on, there is a (very unlikely) race condition. If the Lambda managed to be created and get invoked before Terraform created the log group, Terraform will hit a conflict that only import can resolve.

This also ensures that during a terraform destroy that the graph is cleaned up in the reverse direction and the Lambda is destroyed before the log group. This is potentially important, because the Lambda will re-create the log group when invoked.

Testing

terraform validate is the most testing we can do here.