Scans for factory plug-ins on the application class path. This method is
needed because the application class path can theoretically change, or
additional plug-ins may become available. Rather than re-scanning the
classpath on every invocation of the API, the class path is scanned
automatically only on the first invocation. Clients can call this
method to prompt a re-scan. Thus this method need only be invoked by
sophisticated applications which dynamically make new plug-ins
available at runtime.