Exposing API for other plugins that use optional depends

I want to set up a @Service (let's say Application level) that provides an interface to other plugin authors can call from their plugins.  So for example, if my plugin is com.foo, I want it to expose a @Service called MyAppService so that other plugins can request it *if* my plugin is installed. 

 So I understand that “optional” can be specified from another plugin, call it com.bar in its plugin.xml/depends.  But I'm having trouble figuring out exact how to set this up for development.  And I understand that lightweight plugins will be found simply on a scan for the @Service annotation and I shouldn't need to expose the service separately in plugin.xml for com.foo.

 Specifically:

- how do I “publish” com.foo so that from plugin.xml, build.gradle.kts and anywhere else the plugin needs to be referenced by com.bar, it is “visible”?  Is there a local repo like maven would use to publish working build SNAPSHOTs of com.foo?

- how does “MyAppService” get on the runtime classpath for com.bar?  Does it get added automatically when I successfully make the plugin dependency and publish my plugin (com.foo)?

- is there a template or best practice for how I could build from com.foo and com.bar in tandem so that I could make could changes to com.foo, rebuild, then rebuild com.bar and test my changes in to com.foo in com.bar?