If the index is created for a fragment, the manifest must point at the
referenced fragment plug-in, while the destination should be the fragment
Starting from the provided destination directory, index for each locale will
be placed in a directory with the following path:
The relative directory specified in the index element of the
org.eclipse.help.toc extention will be created in each of the
locale-specific paths (one per locale).
An instance of HelpIndexBuilder can be cached and used
multiple times for different manifest and destination values.