Preinstrumentation does not work for plugins that depend on other plugins

[ogolberg]

E.g. if plugin under test has a compile-only dependency on AGP, which is typical for plugins that need to have some Android-specific variant/source set resolution logic, and the test project has AGP applied, as usual, the test will result in a ClassDefNotFoundError because the plugin under test and AGP live in different classloaders.

The other version of this issue is when the plugin under test depends on a first-party Kotlin plugin, e.g. Kapt. Because of artifact filtering, again, the Kapt plugin will live in a different classloader and will not be acessible by the plugin under test.

To fix this, we would have to bring compile-time dependencies into the TestKit plugin classpath and fine-tune dependency filtering or go back to the agent-based coverage collection after Gradle 8.8.