Gradle plugin marker artifacts as default bundles

If you write convention plugins in separate projects, or write a plugin that depends on another plugin, it is quite common to depend on the used plugins using their marker artifact `:.gradle.plugin:` instead of depending on the actual code artifact.

It would be nice if this plugin would support this pattern out-of-the-box without the need to declare all used plugins as bundles manually.

Start by tracing how manually declared dependency bundles are represented and how plugin marker artifact coordinates are currently handled. Determine the expected behavior for convention plugins and plugins that depend on other plugins; done means marker artifacts are recognized automatically without manual bundle declarations, with coverage for both described use cases.

Written by the indexing model from the issue text.