qapi: Split up scripts/qapi/common.py

The QAPI code generator clocks in at some 3100 SLOC in 8 source files.
Almost 60% of the code is in qapi/common.py.  Split it into more
focused modules:

* Move QAPISchemaPragma and QAPISourceInfo to qapi/source.py.

* Move QAPIError and its sub-classes to qapi/error.py.

* Move QAPISchemaParser and QAPIDoc to parser.py.  Use the opportunity
  to put QAPISchemaParser first.

* Move check_expr() & friends to qapi/expr.py.  Use the opportunity to
  put the code into a more sensible order.

* Move QAPISchema & friends to qapi/schema.py

* Move QAPIGen and its sub-classes, ifcontext,
  QAPISchemaModularCVisitor, and QAPISchemaModularCVisitor to qapi/gen.py

* Delete camel_case(), it's unused since commit e98859a9b9 "qapi:
  Clean up after recent conversions to QAPISchemaVisitor"

A number of helper functions remain in qapi/common.py.  I considered
moving the code generator helpers to qapi/gen.py, but decided not to.
Perhaps we should rewrite them as methods of QAPIGen some day.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20191018074345.24034-7-armbru@redhat.com>
[Add "# -*- coding: utf-8 -*-" lines]

scripts/qapi-gen.py

@@ -5,16 +5,18 @@
 # See the COPYING file in the top-level directory.
 
 from __future__ import print_function
+
 import argparse
 import re
 import sys
-from qapi.common import QAPIError, QAPISchema
-from qapi.types import gen_types
-from qapi.visit import gen_visit
+
 from qapi.commands import gen_commands
+from qapi.doc import gen_doc
 from qapi.events import gen_events
 from qapi.introspect import gen_introspect
-from qapi.doc import gen_doc
+from qapi.schema import QAPIError, QAPISchema
+from qapi.types import gen_types
+from qapi.visit import gen_visit
 
 
 def main(argv):