Refactor and rework several areas of the Gradle plugin:

- Refactor package structure into functional areas and configure each
  area separately via a new PluginFeatures interface.
- Convert BootRunTask to extend RunTask rather than attempting to
  find existing RunTasks.
- Simplify agent integration by no longer needing specific BootRunTask
  code.
- Update the repackage task to consider the `mainClassName` property
  in addition to `springBoot.mainClass`.
- Automatically set `mainClassName` when calling `run` or `runBoot`
  from `springBoot.mainClass` if there is one.
- Ensure that explicitly defined `main` options on JavaExec tasks always
  take precedence.

Fixes gh-547, gh-820, gh-886, gh-912