Update `LaunchedURLClassLoader` so that packages defined from exploded
archive folders have manifest attributes applied to them. Prior to this
calling `package.getImplementationTitle()` would only return the a
manifiest attribute when running non-exploded.

The root cause of this issue is the way that `URLClassLoader` handles
the different URL types. For URLs that reference a jar the manifest is
available. For URLs that reference a folder it isn't. When running
exploded we use a URL that references to the `BOOT-INF/classes` folder
directly. To fix the issue we now attempt to detect when `definePackage`
is being called directly, and replace `null` entries with actual
manifest values.

Fixes gh-21705