Breaking change required: reduce dependency on Gradle internal API

[KengoTODA]

Currently this plugin has several problems caused by dependency on Gradle internal API:

1. Non intuitive usage for Android project (e.g. Unable to use Spotbugs in Android application #90)
   • parent class assumes that sourceSet is created by Java plugin
2. Need to chase breaking change in Gradle side (e.g. Gradle 5.1 compatibility issues #74)

It also depends on the undocumented "convention mapping" API. We're not sure when it'll be removed from the Gradle API.

To make this product easy to maintain, I think we need to remove these dependency on internal API. So, I suggest to do so in the next major version 2.0 because it will break backward compatibility. I'll use this issue to gather TODOs and PRs to realize.

[KengoTODA]

refs #29

[baron1405]

Gradle has the issue gradle/gradle#9196 to convert its built-in code quality plugins to use the Worker API. Once that is done, it can serve as an example to help with this issue.

[KengoTODA]

Achieved by 4.0.0 release