You are not able to use the plugins { } block anywhere except the build.gradle file for a project. You would need to fall back to buildscript { } instead if you use script plugins.
However, script plugins are designed for organizing logic in a project, not for sharing build logic between independent projects. For the case that you describe, a standalone, binary plugin is the preferred approach to share logic between independent projects. Ideally you write tests to verify this logic as well.
Your original post on SO before the edit was much closer to what you should have for the case described, but with a few implementation issues. I’ve provided a more detailed response there on those particular items.