How can I apply a Gradle plugin to itself? - gradle

Question

I have a Gradle plugin which codifies a collection of checks to perform on the codebase. I would like the plugin itself to be subject to the same checks.
Attempt 1
I found part of a solution here, which essentially tries to use buildSrc to produce a plugin which can then be applied from the main project.
This gives me an error someone else had also seen:
No signature of method: GradlePlugins.buildscript() is applicable for argument types: (GradlePlugins$_run_closure1) values: [GradlePlugins$_run_closure1#2e7cad0f]
Attempt 2
This was adapted from a second suggestion in the thread:
apply plugin: new GroovyScriptEngine(
[ file('src/main/java').absolutePath,
file('src/main/groovy').absolutePath,
file('src/main/resources').absolutePath ].toArray(new String[0]),
this.class.classLoader)
.loadScriptByName('acme/plugins/CommonPlugin.groovy')
But it seems like it can't find classes in the same package as the plugin:
Caused by: groovy.lang.MissingPropertyException: No such property: CheckStringsExtension for class: acme.plugins.CommonPlugin
Attempt 3
There was a comment on the other gist suggesting that it might not work if the plugin script contains a package declaration, so I also tried making a trivial wrapper:
import acme.plugins.CommonPlugin
import org.gradle.api.Plugin
import org.gradle.api.Project
class SelfApplyBootstrap implements Plugin<Project> {
#Override
void apply(Project project) {
new CommonPlugin().apply(project);
}
}
And then:
apply plugin: new GroovyScriptEngine(
[ file('src/main/java').absolutePath,
file('src/main/groovy').absolutePath,
file('src/main/resources').absolutePath ].toArray(new String[0]),
this.class.classLoader)
.loadScriptByName('SelfApplyBootstrap.groovy')
But I still get the same error:
Caused by: groovy.lang.MissingPropertyException: No such property: CheckStringsExtension for class: acme.plugins.CommonPlugin