Plugin tests do not compile due to missing Guava dependencies

[carlosame]

Executing the build with Java 16, Gradle 7.0.2:

> Task :compileTestJava FAILED
C:\***\gradle-modules-plugin\src\test\java\org\javamodularity\moduleplugin\ModulePluginSmokeTest.java:3: error: package com.google.common.base does not exist
import com.google.common.base.Charsets;
                             ^
C:\***\gradle-modules-plugin\src\test\java\org\javamodularity\moduleplugin\ModulePluginSmokeTest.java:4: error: package com.google.common.io does not exist
import com.google.common.io.Resources;
                           ^

I'm preparing a PR to add the two missing dependencies.

[siordache]

Thanks for the PR! Fixed in 1.8.7.