Reusing common code(groovy) - jira development

[MarkoRocko]

In script runner I can access my custom groovy packages and make my groovy files reusable.

However, when I am working in a plugin the groovy packages I could access on script runner are not accessible.
Is there a way to expose my custom groovy(classes) packages making them accessible/reusable from inside a plugin or one of my custom plugins?
I suppose I could package them up and snapshot them into a jar file adding them to a classpath and that would work. 
However, I would love it if I could just add the path somewhere but don't know if thats how it works.

I started looking at tomcat and was wondering perhaps if a path can be added there?

Please advise.  :    )