provider/aws: Support import of aws_lambda_event_source_mapping (#14898)
provider/aws: Support import of aws_lambda_event_source_mapping (#14898)

Fixes: #14017

```
% make testacc TEST=./builtin/providers/aws/ TESTARGS='-run=TestAccAWSLambdaEventSourceMapping_importBasic'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/29 00:36:31 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws/ -v -run=TestAccAWSLambdaEventSourceMapping_importBasic -timeout 120m
=== RUN   TestAccAWSLambdaEventSourceMapping_importBasic
--- PASS: TestAccAWSLambdaEventSourceMapping_importBasic (144.23s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	144.258s
```
ashinohara/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,ashinohara/terraform-provider-aws,kjmkznr/terraform-provider-aws,ashinohara/terraform-provider-aws,ClementCunin/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,Ninir/terraform-provider-aws,ClementCunin/terraform-provider-aws,nbaztec/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,ashinohara/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,nbaztec/terraform-provider-aws,Ninir/terraform-provider-aws,ClementCunin/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,ClementCunin/terraform-provider-aws,nbaztec/terraform-provider-aws,nbaztec/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws