Per-repo built-ins from plugins fail to load

To reproduce:

• Create a script plugin which adds a configurable, repo-level built-in script, and install it on Bitbucket (see linked ticket SRBSUP-385 for an example which adds a new repo-level pre-hook; atlas-mvn package from inside the root of the zip will create a jar that you can upload).
• Go to e.g. Script Pre Hooks (or wherever your new built-in lives) in the settings for a single repo and click Add New Item.
• Click on the built-in you added.

Expected result: You can configure the built-in and add it as a new item.

Actual result: No configuration form is visible - the area where is should be is blank.

The reason for this is that, in order to check that a built-in can be made available at the repo level, ScriptRunner has to load the corresponding plugin class and check its annotations. However, the classloader fails, which ScriptRunner interprets as meaning that the built-in should not be available.