Update the Maven and Gradle packaging for war files so that a
`classpath.idx` file is written into the archive that provides the
original order of the classpath, as was previously done for jar files.
The `WarLauncher` class will use this file when running as an exploded
archive to ensure that the classpath order is the same as when running
from the far war.
Fixes gh-19875
8f57f0babb