Previously, JarLauncher considered any entry whose name began with
BOOT-INF/classes/ as being a nested entry. This was incorrect as it
meant that subdirectories of BOOT-INF/classes/ would be added to the
classpath rather than just BOOT-INF/classes/ itself.

This commit updates JarLauncher so that only directory entries with a
name equal to BOOT-INF/classes are used.

Closes gh-5610