db migration commands results in errors when using inline/exploded plugins

When a project uses any plugin as exploded as below

```
grails {
exploded = true
plugins {
compile project(':inline-plugin')
}
}
```

Db migration will try to run the commands on the plugin too, and will fails with error

Try with attached example
[bugwork.zip](https://github.com/grails-plugins/grails-database-migration/files/1104249/bugwork.zip)

run the grails dam-generate-gorm-changelog on the app and u should see the error.

Download and run the attached bugwork.zip example, then execute grails dam-generate-gorm-changelog on the app with the inline plugin configured as exploded. Trace why the migration command runs against the plugin and verify the command completes without the reported error while handling the plugin correctly.

Written by the indexing model from the issue text.