Setting PACKER_LOG doesn't output AWS API requests/responses

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#### Description

When enabling debug logging in Packer, via `PACKER_LOG`, all AWS API requests and responses should be included in that log output, similar to the example output shown in [the AWS Go SDK docs](https://docs.aws.amazon.com/sdk-for-go/v1/developer-guide/making-requests.html#get-http-request-response).

#### Use Case(s)

I'm trying to debug a subtle AWS-related issue with a Packer build. CloudTrail, as usual, is being less-than-helpful, and so I turn to the old standby -- examine exactly what is being sent to (and received from) AWS. I set `PACKER_LOG=1`, fire off my build, and at the very least I've got more data from which to construct a hypothesis.

I could also use these API calls to produce a minimal test case (that doesn't involve waiting for a whole build cycle) so that I can improve my debugging test cycle time.

#### Potential configuration

```
PACKER_LOG=1 packer build ...
```

#### Potential References

* https://docs.aws.amazon.com/sdk-for-go/v1/developer-guide/making-requests.html#get-http-request-response -- how to enable request/response logging in an AWS client.