This should, in theory, bring the dependency as well, but it doesn’t happen.
No it should not. As I have said above in your plugin’s build.gradle file you should add the dependency.
Then in your pom.xml make sure the dependency is added and then Gradle will automatically download the dependency when your plugin is applied to a project.
I would think you’d put api here, for it to propagate to the users. I tried this and it didn’t work.
You do not need api. Those dependencies will be applied as classpath dependencies in the projects that your plugin is applied. Then Gradle should be able to recognize the 3rd party plugin you want to add