I'm facing the same problem (with a different .jar file) and I found a workaround. I know it's not what we are looking for (A way for the plugin to automatically have that .jar file in its classpath when being used by another project), but it works. Add the line

unmanagedJars in Compile += {
    Attributed.blank(file(System.getenv("JAVA_ORACLE") + "/jre/lib/jfxrt.jar"))

to the file yourMainProject/project/plugin.sbt. The same file where you placed the line addSbtPlugin(...) with you plugin.

Hope it works. If you get to find a way to solve it in the nice way, let me know.

Also check this answer, it may be helpful How to generate sources in an sbt plugin?

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