In external scripts (we call them script plugins), plugin IDs cannot be used. Instead, the fully qualified class name has to be used. This is a known bug.
Secondly, externalizing a build script block into a script plugin isn’t supported. (It’s a tough problem, and can’t think of a good way to implement this.) You may have to live with some duplication, at least for the time being. Remember that dependencies specified in a project’s ‘buildscript’ block are visible to all subprojects. Hence, as long as you don’t need dependencies to be available in a script plugin, you just need to declare them in the root project’s build script.