Encapsulation of the information needed by the XMLReader to
read entities.
This class may include information about the public identifier,
system identifier, byte stream (possibly with character encoding
information) and/or the character stream of an entity.
Applications will create objects of this class for use in the
XMLReader.parse method and for returning from
EntityResolver.resolveEntity.
An InputSource belongs to the application, the XMLReader is not
allowed to modify InputSource objects passed to it from the
application, although it may make copies and modify those.