For an example of the problem see 68c0eb4. This PR changed the paths of several plugins, which required manually finding and updating the points of entry into these plugins from other plugins.

Changing a plugin's URL should be considered a breaking change from the perspective of dependent plugins. It's easy to miss a plugin's dependency upon another plugin's URL, but by requiring plugins to publish entry points into their apps we can force dependent plugins to identify and declare this dependency via kibana.json (and properly handle the case in which the dependency is missing).