Fix tests when running from the command line

When loading classes from the jar file, we can't just use the
URLClassLoader since it can not enumerate files in a jar directory.
Restoring the ModuleClassLoader and making all paths relative to the
system class loader (as opposed to relative to the class location).

Change-Id: Ib3f5d12dd5c964d0ba9cc6c5ec9cb556c989e653
(cherry picked from commit 2a4a6c81f8a103be5c48d8a0605a3e4416e8f7f1)