IDEA-287090 update plugin parent classloader when optional dependency is loaded dynamically

[jansorg]

This is a fix for https://youtrack.jetbrains.com/issue/IDEA-287090

When an optional plugin dependency optionalPlugin of plugin mainPlugin is loaded dynamically at runtime, then the features of mainPlugin using optionalPlugin were enabled, but the Java classpath of mainPlugin did not include the classes of optionalPlugin. Further details have been documented in the YouTrack issue above.

The bug is a major nuisance for plugins, which make use of optional dependencies.

This PR changes the parents property of PluginClassLoader to be mutable to allow changing it when an optional dependency is loaded. I can't tell if that's how you'd like it, but IMHO it should be safe because it's volatile and only updated in a WriteAction, AFAIK.

The test only validates the handling with old-style plugins, because AFAIK the new-style module approach is already handling this case, i.e. the module depending on optionalPlugin is loaded when optionalPlugin is loaded and unloaded again when optionalPlugin is removed again. I don't have any experience with the new-style module descriptors, so I may have misunderstood the code.

Thank you for your consideration!