Investigate removing Plugin-local.jar binary from samples

[jaredh]

Currently we commit a local build of the Testify plugin to use in our Sample apps. Committing a mysterious JAR and having it executed in the samples isn't great for a number of reasons, security being one of them.

We should be able to use something like a composite build to substitute the actual plugin module instead of depending on the JAR.

This feedback relates to:

• [ ] The Kotlin library
• [ ] The Gradle plugin
• [ ] The IntelliJ Platform plugin
• [X] The sample code
• [ ] The documentation

[jaredh]

https://docs.gradle.org/current/samples/sample_composite_builds_plugin_development.html implies that this is indeed possible.