Plugin dependencies should be resolved through the buildscript context

[chrisdennis]

Rather than requiring modification of the plugin repository set to allow the RAT libraries to be resolved through a configuration tied to the host project's repositories the RAT libraries should be resolved through the pluginManagement repository set.

More directly, the following build.gradle.kts should work without needing mavenCentral() added to the project repository set:

plugins {
    id("base")
    id("org.nosphere.apache.rat")
}