Previously, each entry in loader.path could only refer to a standard
jar file. Refering to such a jar would add all of the classes in
the root of the jar to the class path.

This commit adds support for referencing a directory within a jar file
that contains one or more nested jars. For example:

$ java -jar -Dloader.path='jar:file:./lib.jar/!BOOT-INF/lib' my.jar

This will add all of the classes in all of that jars in the
BOOT-INF/lib directory of lib.jar to the class path.

See gh-8334