[3.1.8] Main class is not found when it does not extend GrailsAutoConfiguration itself, even if a superclass does.

### Expected Behaviour

I have a scenario where it makes sense to create a superclass to an application's main application class. I have created a superclass that extends `GrailsAutoConfiguration`, as the default main application does, and added some logic that the main application class will inherit. I expect that the `bootRun` task will successfully find the application's main class with this object structure:

```
GrailsAutoConfiguration
|---ApplicationSuperClass
|---Application
|---static main(String[] args)
```
### Actual Behaviour

The `bootRun` task does not successfully find the application's main class. The only way the `bootRun` task finds the main class successfully is when it directly extends `GrailsAutoConfiguration`.
### Example Application

https://github.com/budjb/MainClassSample

Reproduce the issue with the linked MainClassSample application and its bootRun task, then trace how the application's main class is discovered when the class extends ApplicationSuperClass. Done means bootRun successfully finds and launches Application when an inherited GrailsAutoConfiguration is present.

Written by the indexing model from the issue text.