Switch sample plugin to an IT

[jglick]

It would be better to use the Invoker plugin to demonstrate that a plugin can properly consume the BOM than to use the current submodule hack.

[basil]

Even more so now that the dependency tree of sample-plugin has grown so large that it is now sufficiently different from production use cases so as to not catch Enforcer errors that are thrown in production use cases. This would suggest some possible Invoker-based tests: not only a full-tree test that is equivalent to sample-plugin, but also targeted tests for particular subsets of the dependency tree whose components are likely to be used together in practice, for example:

• Frontend library plugins
• Backend library plugins
• Database plugins
• Kubernetes-related plugins
• SSH-related plugins