Preserve XPI external entities

The DTD files in XPI archives may contain "external entities," which look like:

  <!ENTITY % platformDTD SYSTEM "chrome://browser/locale/help/platformStrings.dtd" >
  %platformDTD;

These need to be preserved in output, along with their ordering. There is probably no need to resolve them on import (especially if the referenced file is a DTD file with a chrome path, because it will be parsed anyway).

It's considered good practice to keep the use of these external entities uniform between translations (so if one locale uses an external entity, another locale will also use it). There is nothing to guarantee this, however.

It looks like the parser can catch these external entities by overriding the new_external_entity method in its DtdConsumer. The question is where to put them!