qapi: Record 'include' directives in intermediate representation

qapi: Record 'include' directives in intermediate representation

The include directive permits modular QAPI schemata, but the generated
code is monolithic all the same.  To permit generating modular code,
the front end needs to pass more information on inclusions to the back
ends.  The commit before last added the necessary information to the
parse tree.  This commit adds it to the intermediate representation
and its QAPISchemaVisitor.  A later commit will use this to to
generate modular code.

New entity QAPISchemaInclude represents inclusions.  Call new visitor
method visit_include() for it, so visitors can see the sub-modules a
module includes.

Note that unlike other entities, QAPISchemaInclude has no name, and is
therefore not added to entity_dict.

New QAPISchemaEntity attribute @module names the entity's source file.
Call new visitor method visit_module() when it changes during a visit,
so visitors can keep track of the module being visited.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180211093607.27351-18-armbru@redhat.com>
[eblake: avoid accidental deletion of self._predefining]
Signed-off-by: Eric Blake <eblake@redhat.com>