Plugin does not recognize service level layer

Version Info
Serverless: 3.18.1
Serverless-plugin-lambda-insights: 1.5.0

```yml
# serverless.yml
provider:
name: aws
...
layers:
- layer1ARN

functions:
func1:
...
# no function level layer

plugins:
- serverless-plugin-lambda-insights
```

Expected Outcome:
func1 has two layers, `layer1` and `LambdaInsightsExtension`

Actual Result:
func1 has only one layer `LambdaInsightsExtension`