Adjust how classpath entries manipulated for Java9 support

Adjust how classpath entries manipulated for Java9 support

Prior to this AspectJ would discard ignore the ClasspathEntry
objects built by JDT and just work with the classpath as a string,
driving the JDT FileSystem to rebuild classpath entries again at
a later date using the string. This is more complex in Java9 because
the string representation was losing whether some entries came in
via modulepath.  ClasspathEntry construction for modulepath entries
is non trivial (since the module-info must be processed).

The new version will cache some of the ClasspathEntry objects (those
built for modulepaths) and do more work on the AspectJ side building
classpath entries in general. It now passes these entries to a
different FileSystem entry point rather than the entry point that
takes a string path.