Add new method 'findContainingClasspathEntry' to IJavaProject so it is
possible to find in which classpath a particular resource is located.

Change-Id: Ibc9a9a46000e8e9b91e686c143f3f6427726567d
Reviewed-on: https://git.eclipse.org/r/c/jdt/eclipse.jdt.core/+/185329
Tested-by: JDT Bot <jdt-bot@eclipse.org>
Reviewed-by: Gayan Perera <gayanper@gmail.com>