Previously, DevTools assumed that the TCCL was a URLClassLoader
when trying to determine the URLs that it should examine to determine
the locations that should be watched for triggering a restart. This
fails on Java 9 as the TCCL is not a URLClassLoader.

This commit updates the logic that determines the changeable URLs to
fall back to examining the JVM's class path when the TCCL is not a
URLClassLoader, typically because the application is running on Java
9. This fall back isn't a direct equivalent of the behaviour on Java 8
as the class path of the TCCL and the class path with which the JVM
was launched may not be the same. However, I consider the fix to be
reasonable for two reasons:

1. In reality, the class path of the TCCL and the class path with
   which the JVM was launched are the same.
2. There appears to be now to get the URLs on the class path of the
   TCCL on Java 9. There is a URLClassPath field, however Java 9's
   access restrictions prevent us from using it even if we resort to
   reflection.

Closes gh-10454